And at a large corp you can get laid off just as easily. Any business can toss you out at a moments notice. It's not unique to startups.
At least with a startup you are going into it knowing that you have a higher probability of thing going south financially. With a big company, you might not get any warning at all.
The startup can go bankrupt any moment, thats a big deal. You get crappy perks, often poor benefits.