No, MS just wants to expand their monopoly, but "MS buying most common browser, after antitrust laws already forbid internet explorer" doesnt ring so well.

Existing userbase

They don't want to buy the browser. They want to buy the brand and the users. Chrome makes up over half the browser market. Think of all the data they could extract from Chrome users. It would cost significantly more to fork chromium and grow the user base to a point where they could extract anything valuable from them, and that's assuming they'd be successful enough to make it happen.