This question already has an answer here:
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.