One possibility is that, if you get in earlier, as the company grows you're more likely to be promoted to more senior positions, which might be good for professional development. Relatedly, you're more likely to work on a larger variety of things since there isn't enough manpower to let you specialize, which can help develop a larger skillset.

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).

The whole point of OPs remark was the payout is not the reason to work for a startup. (precisely because most startups never end in a payout)

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.

I took a 1/3 pay cut to work at a startup because I believe in what the company is trying to do. That's the only reason I keep doing what I do, because the long term goal for the company is in line with my beliefs. Even though I commute for 2+ hours each way, work long hours, and am on call 24/7 for the last 3 years. At some point it may become too much, or another offer might be more interesting, but at the moment, I believe very much in what the company is trying to do.

fun, interesting idea and tech, might be more fun to work for a company pushing the edge of machine learning, or working in VR or that would let you use haskell or rust. The startup might be started by a group of friends/colleagues that you love working with, it might be 2 miles from your house. They might be the only one willing to hire you and take a chance at you. You are willing to GAMBLE and accept that it might fail and all you will have is the pay you got while employed. There are tons of valid reasons for working for a startup, and what else is wrong with making someone else rich? Not everyone is an eagle, some people are little birds that must find an eagle's back to climb upon so they can soar higher than they ever can on their own.

Not all startups are like that. The one I work at (in San Diego, not SV) has a better work-life balance than a lot of non-startups I know. I very rarely work past 5pm.