To be fair, not everybody is interested in running a company, and doing all the "other stuff" that it entails beyond writing code. Also, Google provide great resources and infrastructure, which it would be hard to replicate (assuming the hypothetical project in question required Google scale infrastructure).
I can see where that would be true for some people, but I doubt it's universally true. And it may even be that it just makes more sense to do the idea inside Google than doing it independently anyway. Again, look at the resources and infrastructure Google already have assembled.
And some people may have just bought into the Google mission / vision / whatever, and / or just feel a sense of loyalty to GOOG.
Anyway, I don't mean to suggest that it never makes sense to quit and do your own thing. I mean, hell, I probably would, as far as that goes. I'm just saying that there are some legitimate reasons why some people might prefer to just work within Google than quit and run off to start a new business of their own.