[munificent]

> it's because bad NPC AI is by design, so players can learn to overcome them and succeed.

That's part of it, but there are other factors too. The more complex the AI, the harder (i.e. more expensive) the game is to tune and test. Game producers and designers are naturally very uncomfortable shipping a game whose behavior they can't reasonably predict.

This is a big part of why gamers always talk about loving procedural generation in games but so few games actually do it. When the software can produce a combinatorial number of play experiences, it's really hard to ensure that most of the ones players will encounter are fun.