Umm, I've never worked in a startup, but if I do, a huge motivation would be make money. Risk is startup failing, not it getting acquired for good money. In the end, if all I'm getting is standard RSUs and salary bump, I might as well will join the established company at first place. I understand some people really do love to work for startups, but I think many don't and regardless potential to make money much more than standard job in future is always a motivation.