AI doesn't have to only try to win to be good. It has to give the perception of competition. Perhaps just scaling behind the scenes based on how the player is progressing
I say this not because I think it's an especially noteworthy or important objection, but to echo GP's point, that it's very hard to find AI that suits everyone, and it's not just a matter of difficulty.
And by having different AI at different difficulty levels, I think you actually can have AI that suits everyone. But I don't think most game companies like developing several separate versions of AI. They just want one that's superficially "good enough".