Lets do a comparison from around the 17th century:
France: 1600 - 20,000,000
1670 - 18,000,000
1700 - 21,000,000
England (proper): 1600 - 4,110,000
1650 - 5,310,000
1700 - 5,200,000
Sweden: 1570 - 900,000
1650 - 1,225,000
1700 - 1,485,000
To put it simply, other countries had more land suitable for agriculture and a larger population to draw from during this time period. Gustav II Adolf did make strides in modernizing the military but if Sweden did try to create a Baltic Empire of sorts then they would be hard pressed to hold its possessions. Yes, France did help Sweden in the Thirty Years War but how long would that tenuous alliance hold?
During the Thirty Years War, Sweden only landed roughly 13,000 troops in northern Germany and another 24,600 were left to defend Sweden. The other 33,000 was contributed by Saxony (early on in Sweden's involvement), Scotland, and France (during the later stages of the war). France provided money to recruit mercenaries. I have no doubt that Sweden could field more troops but with the wars against Russia, the Polish-Lithuanian Commonwealth, and Denmark prior to the Thirty Years War, Sweden ended up needing funds.
Also, as evidenced by the sinking of the Vasa and the Kronan, they needed to make adjustments to how they built their ships. I understand that the art of shipbuilding was still evolving but if they are to have a monopoly on Baltic trade then naval power is essential.