Actually creating smart AI is supposedly easier. The problem is creating AI that doesn't completely demolish the player while still challenging them.
The problem with difficulty sliders is that for a lot of games increased difficulty=numbers bloat. Making me tediously shoot a boss 500 times while they 1 shot me might be technically more difficult, but it's not in a satisfying way. Compared to something like Sekiro that's fine tuned to be difficult but satisfying when you master it, I much prefer the latter.