The problem with established companies like Google is that all of the cool projects are already taken.

That, and that they claim ownership over whatever you create, including stuff you do in your off hours, and any ideas you may have had.

But the lack of opportunity is the big one. There's just too many people who've been there too long, with their own projects to work on that overlap with the work you'd want to do, to have any shot at actually doing groundbreaking work.

Interesting work? Maybe. Groundbreaking? Not gonna happen.

As an example, I work on pretty much the same problems as Jeff Dean and Sanjay Ghemawat, albeit with a radically different approach. Would I be able to continue that work at Google? Extremely doubtful, again, because all of the groundbreaking work is taken.

(Note: Jeff Dean and Sanjay Ghemawat are awesome, I'm not trying to imply that they aren't up to the task or whatever. Far from it. Just that there's already really good chefs in the kitchen at Google, so there's no point in joining in the fun.)