So if you're not going to do it for the payout, which was the main reason to do it, then why do it? Why put yourself through the crap and the unpaid overtime and the stress and the subpar pay for the chance to make someone else rich?
Whether they balance unpaid overtime/stress, however, can be iffy. If you can get in without the bad parts, though, it can be good (but that's less start-up and more small company).
There are many other reasons why a startup would be attractive to some people. Professional autonomy, personal growth and learning, different work culture to name a few. For many people, once you're earning "enough" to cover your expenses the pay difference between employers is worth the trade off.
In the end, unless you're founding the company yourself you're always working "to make someone else rich". That's completely irrelevant to your personal choice of employer.