Why don't large commercial airplanes have electric motors in the wheels? The cost of jet fuel during taxiing can be a pain. Concorde in particular had very large fuel expenditure for taxiing.

So why not put electric motors in at least 2 wheels to drive the plane that way? They would not have to be the most powerful motors in the world---only strong enough to get the plane up to a few miles per hour, or normal taxi speed.

EDIT: I don't think this is a duplicate of this other question. If anything, the savings cited in the answer of that question seems to imply that my question, asking why it isn't implemented, is actually a pretty good question.
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