Founding yet another tumblr-for-cats and selling ads isn't ambitious either; you can do mundane work at startups just as easily as you can in a big corporation. You can do ambitious work at both of them, too. For kids who don't come from a wealthy family, there's something to be said for building up a minimal safety net early on by doing ambitious things in a relatively stable large firm. It's not the only way, though (obviously).