Because they have to. The rest of you have been busy voting the government more and more power which means google has to lobby to keep their competitors from using that power to kill them. The only way to reduce the power of lobbyists is to reduce the power of the government. I feel like I'm the only one who sees that truth.
A better question is "Why does Google lobby so little?". Given the payback, lobbying is a very efficient way of companies to influence the markets that they play in.