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.