[PeterisP]

This approach creates a mismatch between singleplayer and multiplayer modes - this means that playing against a computer opponent rewards/requires different strategies than a human; a challenging computer opponent has more income and units but poor usage of them, while a similarly challenging human opponent has less income and units but uses them very differently; so playing against challenging computer opponents doesn't help you improve against other players but possibly is even counterproductive as you learn to adopt strategies that are bad in the other environment.

[the_af]

True. I don't know many videogames in which playing against the computer really helps you against human opponents.

I don't know whether a more capable non-cheating AI would help though. Not unless it specifically imitated how a (good) human opponent would play, which I guess is an additional and difficult to implement constraint.