Why doesn't everyone just exercise as soon as they join a company? At least at earlier stage startups, it seems that the amount of salary offered as compansatio

Why doesn't everyone just exercise as soon as they join a company? At least at earlier stage startups, it seems that the amount of salary offered as compansation is at least an order of magnitude more than the amount of options. Given this ratio, it seems like most employees should have enough in liquid savings after even a few years to avoid taxes on the appreciation between stock grant and vest times. Of course, there's the possibility that a startup will tank, but even in that case, losing out on having bought stock seems much smaller than the opportunity cost of not having worked at a sure-bet tech giant.