Why don't large commercial airplanes have electric motors in the wheels?
In fact, this is currently on test.

This test is conducted by Airbus, Safran and Honeywell for the Airbus 320 familly on this aircraft.



An engine is installed on the wheels of the main landing gear



As you might know it take long time and lot of money to certified a system in aeronautics, this is why it's not in use yet.

I other hand, you need to run engine in idle for about 5 minutes in order to warm up the engine and stabilise systems (hydraulics, gyroscopes, etc)

You can find more here : http://www.greentaxiing.com/

Edit 1 : edit to add @Ben elements