Honestly if I am going spend my years on building something from scratch and sacrifice so much for its growth, I would like to have some say on its further development and future course and a fair share of its success too (acknowledgement and financial wise). That rarely seems to happen in startups, apart from top engineers and executives. Even if you come up with ideas that transform the company, best you get is some sort of bonus and maybe sth you claim you have done in your next job interviews.
If you want such rewarding stuff then work in the academia for somewhat a better salary and much more freedom and ownership of projects you develop and supervise (even as a postdoc).