I've wondered the same thing. It's not necessarily an either-or situation. If Google's willing to spend X billions of dollars on defensive patents, why not spend even a fraction of that on lobbying for software patent reform. If any company has the will and the means to do so, it would be Google.
While you can certainly throw some money behind a favorable legislation, I don't think it's as easy as making out a check to Washington DC and have your words be law. Politics are still a complicated, slow and cumbersome process.
