My dad has worked at the same company since he dropped out of college around 40 years ago (he runs it now). I do think our generation's (well, I'm 32, don't know about you) job goals are a lot different and a lot more conducive to short-term jobs, and I don't think there's necessarily anything wrong with that. I tend to place maximum value on doing real work and self improvement, and lots of meetings and doing the same work for years aren't the best way to achieve these (fundamentally unachievable) goals.
But even more than that, I think a major problem is that startups are companies that are by definition changing ceaselessly. They're either growing or belt tightening, adding new people who need to be managed even if the team is still functional. The emphasis goes from core product building to scaling and optimizing. Etc.
So, it might be sort of you, but it may also just be the nature of the work. Don't take it too hard.
I have a friend who's... hard to work with? He changes jobs a ton. But his strategy has evolved from 'join a team then get frustrated' to 'join a project, know what you're going to work on, add a ton of value, then maybe find another project at the same company, or not.' It's kind of mercenary but can be really valuable to both parties if you play it right, and keeps him from feeing miserable.