Some people are choosing to do so because for them the experience is worth it.
It's like "working" by going to an apprenticeship. Your salary is lower, but you learn a lot.
If you're going to start your own business in the near future, then it might be worth it to work for a startup even if its a huge pay cut all things considered.
If you are working 80 hrs/week churning out code to realize someone else's "vision" who treats you like a slave not a partner, then what are you learning? Apart from not to get suckered again.
It's like "working" by going to an apprenticeship. Your salary is lower, but you learn a lot.
If you're going to start your own business in the near future, then it might be worth it to work for a startup even if its a huge pay cut all things considered.