There are many problems with "Arab unity". First off there are the religious divisions within Islam of which Sunni/Shi'a are only the most well known. Then there are the issues of what do you do with "racial" Arabs like the Copts and Druze who are religiously outliers. There are also regional animosities - Arabs of region/country "A" have feelings about those of region/country "B" (they're lazy or pushy or uppity or whatever). Finally, and perhaps most importantly since none of these countries do more than lip service to public desires except when forced, who gets to be the boss? Somebody has to be the president/premier/caliph or whatever, and none of the Arab leaders have ever shown any tolerance for being other than number one. Perhaps this is in part because local number ones tend to make sure that any popular number twos (or threes or fours) never get a chance to be number one. Finally some Arab countries have a high per capita GDP because of low populations and oil revenue, others much much lower due to large populations and no oil (compare Kuwait and Egypt for example) - so in a "unified" Arab polity, following the money.