I have lots of 25-year-old friends who would like to start a startup but decided against it. I'm trying to convince at least one of them to reconsider, since my cofounder quit yesterday and that leaves me a single founder. Their reasons are pretty varied, but include:
1.) One has Marfan's syndrome and needs to work at a place with guaranteed health insurance to cover his medical bills.
2.) Two would like to start a startup in the future, but feel that they don't currently have enough experience. This was also my reason for not founding a startup straight out of college.
3.) One was always interested in startups, but his family background has steered him towards law school, and so he doesn't have the technical skills to start a tech startup.
4.) One has a wife and two stepkids to support.
5.) One (my former cofounder) wants to start a startup, but got into Harvard Business School and figures a bird in the hand is worth 10 in the bush.
6.) One likes working for small companies, but enjoys his work-life balance too much to take the plunge and actually start one himself. He also doesn't desire the financial rewards that come from a successful startup all that much.
7.) One couldn't do it because he's on a student visa and it was doubtful that the visa would let him co-found a company.
They're all good reasons, and you can see that they're a lot more varied than them all being caged animals.