Besides the fact that a lot of these people do it for passion and not to make a company that will just be bought--there are two reasons Google or a similar company would buy them.
The first you mentioned- gaining the users and scale. The other is gaining the skills, wisdom and creativity of these startups...I am sure that is why Google bought dodgeball, when they could have easily made it. It is why Yahoo has not just bought Flickr, but moved the owners into their creative/startup area.
Incorporating startups can help these giants keep an entrepreneuralist mindset. Also, how much does it cost them to create some of the companies versus buying them and getting the earlier mentioned benefits?