In my completely unqualified and baseless opinion, I think the reason machine learning in games isn't more popular, is that ruthlessly difficult enemies that learn and adapt to the player just aren't.. fun. Level playing fields between characters and enemies just don't work. For example, the Dark Souls series is generally known for being "difficult but fair", however enemies are not smart by any means, they ultimately have a very limited number of attacks and very basic AI that consists of "Wait until player is in range, Run in a straight line toward the player until in attack range, attack". Even with that simplistic AI its pretty difficult and enjoyable.The player is meant to triumph over hundreds of enemies, its the player that learns and adapts to a static set of obstacles. If enemies in Dark Souls learned and adapted to the player, it would go from difficult to impossible very quickly and would not be enjoyable.

The purpose of AI in games isn't making something thats impossible to beat, its making something that is ALWAYS possible to beat while remaining fun and engaging.