You should note: founders do make a salary which lowers risk. We have to eat and pay the bills somehow. So the opportunity cost you speak of is simply the net loss of potential salary - startup salary. Founders probably take nothing in a year one, something small in year 2 (50-80), and year 2+ is variable.

Another consideration is you learn a lot more than you generally would at a big company. There's a lot of value in knowledge and many (despite it sounding corny) deem that as invaluable--worth forgoing 100K/year in salary.