Because the billions they make from search aren't enough. Of course they are free to do whatever they want, and you are even allowed to hide those ads (or use open source version), but I think there's a limit where it will get too much.
If it bundles in advertising then is it really free? Consider how much money is spent on ads - they clearly have an effect, and given the obscene amounts of money spent, probably not a small one.
So what is the impact of advertising? We buy more things because someone has convinced us that we need them, become more likely to get into debt and save less as a result. I cannot consider this to be free in good conscience. I'm not the only one who sees it this way either -