It's very hard to leave, even if I find a more intellectually rewarding job in a different place, for three reasons:
1. I have un-learned any skill that I may have had that permitted me to operate in a cut-throat workplace. I'd probably suffer through a steep learning curve if I moved to a company where people are mean.
2. I now know that you *can* have a nice career in an collaborative and cooperative environment, and that that's even healthier and more productive than the cut-throat workplaces. So I'd be constantly reminded of that if I ever left.
3. Stock vesting is set up in a way that makes you want to stay to vest all the stocks you're getting, and every refresh adds more stocks that will vest over 3 years, such that you always have stock coming in soon that you don't want to leave without :)