Weren't they? Casimir the Great's sister, Elizabeth, was married to king of Hungary. Her son, Louis, was king of Hungary and Poland. Her granddaugher, Jadwiga, was crowned as king (!) of Poland.
It's not that Piasts weren't interested in dynastic marriages. Poland was freshly reunited at the beginning of XIVth century, so earlier no bigger dynasty was particularly interested in marriage with some Piast prince. It changed after Poland was united by Władysław Łokietek (The Short) and especially after rule of Casimir, but Poland had no so many royal children to marry them off.
Habsburg indeed were excellent in dynastic games, but who says that Piast couldn't play that too?
Oh, and I found another, longer and modified version of my post for thread Children that could've been
In 1370 good King Kazimierz dies, mourned by all his subjects. But there is a light in the darkness: his son, 25-years old Władysław seems to be as talented as his father was. He inherits a strong and well organized state, with wealthy towns, brave knights, full treasury, and Cracow Academy to educate future generations. Poland is also well respected in Europe. However, the situation is not so good as you may think. The Teutonic Order still has its powerful state on Poland's northern border (occupying Danzig and East Pomerania), Bohemia controls most of Silesia, and in the east...well, Lithuania is a pagan country.
But a good ruler makes one problem solve the other. Young King Władysław starts secret talks with Lithuanian prince Algirdas. They have a common enemy – the Teutonic Order. A year later, Algirdas decides to convert to christianity and baptism his subjects. It is a great diplomatic success for King Władysław. With one stroke he gains an ally, support of the Church (as a man who helped convert so many pagans), and makes Teutonic Order...redundant. After all, the Knights of the Order (known in Poland as Crossmen) were supposed to fight the pagans, not freshly baptismed christians.
Using his popularity, King Władysław starts a diplomatic offensive against the Crossmem. His embassadors show at the papal court together with Lithuanian noblemen, who humbly ask for accepting them to a family of christian states and swear to pay very handsome tribute to Roman Catholic Church. Polish lawyers, educated in Cracow, represent king Władysław in another trial against the Order (in OTL there were many of them – Poland usually won, but the Order didn't respect the verdicts). They demand that the Crossmen give back the territories taken from Poland in early XIVth century: Pomerania and Danzig. The Order manages to prolong the trial, but its situation is not good.
But King Władysław doesn't concetrate only on one thing. He wants to achieve what his father wasn't able to do: reunite all Polish lands from the times of prince Bolesław Krzywousty (Boleslaus the Wrymouth).. including Western Pomerania and Silesia. Thanks to his influence and diplomatic talents he manages to convince Pomeranian princes to declare themselves Polish vassals. It proves to be not so hard, because the princes are affraid of Brandenburg and Teutonic Order. Soon Poland once again has a foothold on Baltic shore, and communication between Brandenburg and the Order is broken.
Silesia is much bigger problem. It is controlled by Bohemia: Silesian princes are vassals of Luxemburg dynasty. However in 1375 king Władysław attacks: his armies push to Wrocław (Breslau) and Świdnica (Schweidnitz). The Teutonic Order tries to intervene on Bohemian side, but its actions are paralyzed, first by Polish-Lithuanian-Masovian-Pomeranian forces, later by papal nuntius. The Crossmen lost the trial, and any military action will be considered by pope (i.e. The Church) an act of disobedience or even a personal insult. If the Order doesn't stand back, it might even get disbanded and excommunicated. The Crossmen appeal, but for some time they must stay quiet.
Meanwhile, Władysław fights Charles IV, King of Bohemia and also...Holy Roman Emperor. Old Emperor is very sick and has no strength to personally lead his armies. Władysław is still young, healthy and full of energy. He is not a great tactician, but a masterful strategist and diplomat. Moreover, he has a secret weapon: his childhood friend, Gniewosz of Szamotuły, who proves to be a brilliant commander. Gniewosz leads a surprising raid and manages to capture Wrocław (Breslau). Next two years are full of minor skirmishes and short truces, but there is no major battle. Charles tries to get support from the rest of Holy Roman Empire, but fails because of great peasant rebellion (supposedly provoked by Polish spies). German princes also use an opportunity to conspire against the Emperor.
This ends in 1377, when Brandenburg attacks Poland from west, while Bohemian army attacks from south. Both attacks are poorly coordinated. Brandenburg forces are delayed by Pomeranians using guerilla tactics. That gives Gniewosz time to deal with Bohemians. In the Second Battle of Wrocław (Breslau) Polish army surprises and defeats the Bohemians. After victory Gniewosz doesn't wait to gloat, but hurries against Brandenburgians. Reinforced by Pomeranians, Masovians and even a small Lithuanian contingent he meets the enemy near Polish town of Kościan. Using a fake retreat tactics Gniewosz leads Brandenburgian army into a lethal trap. During the battle Venceslaus, emperor' son, is killed.
The emperor is shocked by his son's death. What is worse, Hungary seems ready to join the war – on Polish side, to gain as much as possible without much risk. Charles IV asks for peace.
Władysław's conditions are harsh. Silesia is to return to Poland, except of Duchy of Schweidnitz, which remains independent. Brandenburg looses the New March, which is annected by Poland. Bohemia has also to pay a lot of money. Luxembourgs will never forgive Poland such humiliation.
However, there is nothing they can do. In 1378 Charles IV dies, and his heir, Sigismund, is only 10 years old. In Holy Roman Empire starts a long period of internal fighting.
Triumphant Władysław returns to Cracow, welcomed as hero together with his Crown Marshall, Gniewosz. But Poland is not totally safe yet. The Teutonic Order still remains very serious danger. However, Poland suffered to many losses to start a new war. Meanwhile the Crossmen turn against Lithuania, invading Samogitia. Lithuanians defeated in two major battles use guerilla tactics. Polish diplomats do their best, but the Order is rich and can afford a lot of bribes. Poland officially declare, that the Crossmen are heretics, because instead of converting pagans or helping christians (as they are supposed to), they are only interested in their own power. The Order, on the other hand, claims that the baptism of Lithuania was only a trick, and Lithuanians are still pagans. Finally, in 1383, a truce is declared.
The diplomatic conflict lasts about 15 years. Lithuania and Poland keep preparing to another war. So does the Order. It costs them a lot, but Order's subjects suffer more from high taxes. That makes Order very unpopular.
Unfortunately for Poland, in 1398 king Władysław dies. His son, Bolesław (Bolesław) is only seventeen years old. The Crossmen decide that this is their last chance, before Poland becomes too powerful. They violate the truce and attack Polish land of Dobrzyń.
Bolesław, called Wilk (The Wolf), is truely his father's son. With help of Marshal Gniewosz he stops Teutonic army from crossing Vistule near Włocławek and calls Lithuanians and Pomeranians for help. Since it will take some time, to gather forces, he agrees to another truce, which is supposed to last 3 years.
However, Bolesław doesn't intend to wait so long. In 1399 he is the one who violates the truce and with surprising attack regains Dobrzyń. There he is joined by Lithuanian forces led by great prince Jogaila and rest of Polish and Silesian knights. Allied army quickly pushes towards Chełmno (Culm) and Toruń (Thorn). In Toruń citizens of the town overthrow Teutonic garrison and open the gates for Polish king. Culm falls after one assault.
Konrad von Jungingen, the Grand Master of Teutonic Order, gathers powerful army and goes against Boleslaus. Both armies meet in the battle of Kwidzyń – the greatest battle of medieval Europe. Polish-Lithuanian-Masovian forces practically anihilate the Order, again thanks to masterful command of Marshal Gniewosz, who unfortunately is mortally wounded. In his deathbed he begs Boleslaus to finish the war quickly – and that is what Polish king does. Lithuanian prince Jogaila wants to give their troops some rest, but The Wolf convinces him to go further. Meanwhile, in the battle of Stargard, Pomeranians defeat troops from Holy Roman Empire going to help the Order.
Allied army reach Malbork (Marienburg), the Order's capital. It is a powerful fortress, but poorly defended – most of the knights and soldiers died in Kwidzyń. After one week of siege Polish and Lithuanian forces capture the castle. The pitiful remains of the once proud Crossmen beg for peace.
Actually, Boleslaus wants to finish with them once and for all, but the Church wouldn't be happy with eradicating whole religious Order. The peace is signed in Toruń. Lithuania regains Samogitia and the Order promises to support Jogaila, who wants to be crowned as a King of Lithuania. Poland gets East Pomerania with Gdańsk (Danzig), Malbork and land of Chełmno. The Order is also forced to pay an enormous contribution to victorious rulers.
Boleslaus is overjoyed. He finally reunited all lands that used to belong to Piast dynasty. After long and heavy wars, it gained many territories (incorporated or as vassals) and earned a lot of money. Polish borders are finally safe: in south there is friendly Hungary and not-so-friendly-but-too-occupied-with-other-things Bohemia. In south east Polish lands are protected by vassal Moldavia. Allied Lithuania shields Poland from the east and the Order is only a shadow of its former glory. Internal situation is also quite good. Boleslaus doesn't have to give his knights any special privileges (In OTL Jogaila assured in this way Polish throne to his son), so king's power is noticeably stronger