This comment will sound a little confrontational, but here it goes. Why would anyone competent enough to work at one of these companies choose that path instead of being a founder?
You can titrate the amount of startupness you get in your job by the size of the company you join. If you're the first person hired by a startup with one or two founders, you'll probably be a de facto cofounder. Whereas if you get hired by a startup with 30 people you'll have a lot less risk and stress, but it will probably still be more interesting than going to work for a big company.
Another reason is that luck (usually in the form of timing) is a big component of startup outcomes, so in some cases it can be a better bet, measured both financially and by how much effect you can have on the world, to join a startup that is obviously taking off than to start your own. All the first several hundred employees at Google, to take an extreme example, probably made more money and had more effect on the world than they would have on average by starting their own companies.
Plus you need more than ability to start a startup. You also need an idea and probably a cofounder. Joining an existing startup can be the easiest way to get both.