At Google, if a product becomes successful, will get tons of resources and people thrown at it. Imagine you’re working at Google and you have 20% time. Will you keep spending that time on a boring project that isn’t very cool? No, you will want to join a cool project like Instagram that’s getting love around the world and getting tons of adoption. If the Instagram team were at Google they’d have to deal with tons of emails and folks hanging outside their cubes just to try to participate.