Besides everything that has been already mentioned, there's another glaring reason Google would be unwise to do this:

Antitrust.

Google is already being considered for an antitrust investigation by the DoJ. This makes sense, given their dominance in search and the tremendous network of complementary products in their massive ecosystem. (Note that I am not asserting that it makes sense for the DoJ to take regulatory action against Google -- merely that it is reasonable for them to take a closer look).

With a carrier on their hands, in addition to a device manufacturer (Motorola), a dominant mobile OS, search, AdSense, Fiber, tons of spectrum, YouTube, etc., it is unlikely they will be able to avoid antitrust regulation.