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.