>the only reason I jump straight into the multiplayer when I try a new game is because the single player AI, at every difficulty level, in almost every game, is dumb as rocks.

There's more to it than that for me. No matter how good the AI is, shooting at AI feels empty and dull compared to shooting at actual people. Not only are you assured cunning, but also someone who hates your guts and lets you know in chat! I suspect many others feel that way too, deprecating singleplayer in favor of multiplayer from the get-go.

>Making the AI "way too fucking smart" at insane/hard difficulty and then dumbing it down seems way, way, way better than hitpoint modifiers.

It sure does. Have you ever played Civilization? The AI is always dumb; the only way to make the game harder is to make the game fundamentally unfairer. In this case, making the AI smarter is a much harder problem than in a shooter game though.