I think it's less about being underpowered temperature wise and more in the results. You have to fiddle with a few more settings to get the results you want with an oven, especially when trying to make things crispy (or at least I do). Air fryer simplifies it down.
I know nothing of the technologies involved but I suspect the air fryer makes things easier because it's easier to get the desired effects in a small heating area (the basket) than the oven.