Microsoft and Apple don't want Google to have defensive patents (patents that both of them know that they infringe on in droves, otherwise they wouldn't be concerned about them).
They should have worked to remove all of these patents off the table as cheaply as possible. The available cash that MS+Apple simply dwarfs what Google has.
Dwarfs is a gross exaggeration. Further both Apple and Microsoft have shareholder responsibility. Microsoft has already made questionable judgment calls to maintain the upper hand -- billions of dollars in patents to make a few hundred million a year in extortion fees, while empowering their own primary competitor (Apple)? It's extraordinary. Apple shareholders are going to start demanding their cash back if it looks like one giant ego pile.
And Google can't quietly buy patents because the people selling want to get as much as possible for it. Well except for IBM who just wanted to stick a gank in Oracle.
The fact that their current IP strategy seems to be to buy IP is absurd.
Many of the patents that Microsoft and Apple are using against Android precede the mobile explosion. They are older companies, having had a long time of big profits to pay people to sit around submitting patent applications.