Putting together what a bunch of different people have said, it's not just this (although it's this too). It's that you get promoted by producing a high profile, popular product. After which you've been promoted and you're working on something else, and so the product you got promoted for gets neglected, languishes for a bit and then dies.

The problem with this idea is that once the project becomes a product or services of Google's, then it is supported by official paid working hours of its employees.