Except that many of them did create separate states - places like Morocco, Algeria, Oman, etc. have been more or less autonomous or officially independent for many centuries and up to a thousand years. It's one thing if you're talking about Iraq and Syria, or even Saudi Arabia and Jordan, states in which several separate regional identities were combined into one (and I think both friends and enemies would agree that the Palestinian nation is also a rather recent phenomenon, mostly shaped in the first half of the 20th century). But others have a much older national identity, they just have the wider Arab identity in addition to that. Sort of like the relations between the various Spanish-speaking Latin-American countries. Not that that stops them from having minorities which may be in opposition to this national identity.

I can agree with some of that, and certainly the borders of Syria and Iraq and Jordan are pretty arbitrary. I just think you're too black and white about it. Sunnis and Shias have a long history of violent clashes, but so did Catholics and Protestants, once, and in both cases it's often ambiguous whether the religious differences were the actual reason for a war or just a pretext. It doesn't mean that a nation with significant numbers of both is necessarily doomed to failure, it's just a question of political leadership choosing to reconcile, or to divide and conquer. The Kurds could have had their state already at Sevres, and looking at the situation today, it makes sense to give them one, but some of the four nations sharing 'Kurdistan' are or at one point were making decent steps towards a solution involving mere cultural autonomy within the existing borders. To take the most obvious example, Erdogan's Turkey, it's not some kind of historical inevitability that he went back on the warpath and ruined the chances of a lasting peaceful solution for Turkish Kurdistan (in the short term, at least). It's the result of political choices that he and others made, and for which they should be held accountable. Same thing with Assad's and Saddam's divide and conquer tactics in which they played off Sunnites against Shi'ites and Arabs against Kurds in order to maintain their power.