Google is not the only company who accumulated a patent portfolio to be used defensively. If they start suing people they can expect to be sued back. It would also potentially ruin Google's relationship with other companies who would be reluctant to work with them if it could lead to a chance to be sued: e.g. If you sell some technology to Google to use they can discover that it has something that looks like something that they patented, which is enough grounds to sue you. This is too much risk for companies who don't have the money to mount a legal defense, even if they don't actually violate any patents.

The current state is sort of a patent cold war where both sides have weapons but they don't use them. So we get the advantages of patents (disclosure) without many of the potential disadvantages.