A. Sell your phone to VZW/ATT/Sprint, who are all willing to pay only a couple million dollars.
B. Sell your phone to T-Mobile, who are willing to pay $50m?
Obviously, the above numbers are made up. But you get my point, right? Why would Google sell their phone to VZW/ATT/Sprint when they can get more money from T-Mobile? Financially, it makes no sense.