Sure, Google also wants to have a stable client side for their javascript-based software, but the reality is that those programs don't make them the kind of money that search advertisement does. With Microsoft controlling the browser, they could conceivably release an ad blocker in IE8 and turn it on by default. Google would be in serious trouble very fast.
I'm not trying to say that this will happen, just that Google's being smart and mitigating risk. Until they start making serious money from their office applications, however, creating their own browser seems to me to be a defensive move, not an offensive one.