I think it's because equity creates a belief in young workers that the company is "theirs" -- but after you've realized that startups are like any other job where you trade time for money while implementing the Highest Paid Person's Opinion, you lose that passion quickly.
Incidentally, there's a bonus coming up for a major milestone and I'm seriously considering handing in my two weeks notice after the direct deposit clears. The CEO tried to sell the whole equity angle, but I honestly don't see any resume improvement potential here and that's all I really care about.
I've worked at companies that gave a shit and I just don't think this is one of them.
The problem comes when I stick it out for too long at a place that does not share my core values. In my experience it is a losing battle. Trying to match what I believe in with a whole company is really tough. I feel burnout is to be expected at times during my career while I figure things out. A little break, some consideration, and new goals help me move forward and find work I can enjoy.