Because antitrust law is made of words that mean things. There is no field, anywhere, in which Google has the monopolistic position necessary, or has been shown to be in an oligopolic group necessary, to invoke antitrust law against the company. Not search, not ads, not mobile, not e-mail, not anywhere. They have competition, and healthy competition, in literally every field.
I answered the breathless and silly question you asked here instead of punching in your surely-not-Google search engine of choice with exactly the level of seriousness with which I regarded it. HTH. HAND.