AI complexity directly leads to slowing things down or thinning out monsters. It simply has to.
making enemy AI better is easy. If a dev wanted, they could make an little level 1 enemy impossible to beat. Of course, the ai would take so much computation that you wouldn't see another enemy on the map.
Most players simply want easier ai with more mobs. That is just how it is. They don't want to spend 10 minutes or more fighting every single enemy the see.
Look at GW1. They didn't make the ai better for hard mode, they added superficial things to make it harder. Now it depends on how they designed their ai systems at the beginning, but they could've chosen to just make the ai better, but probably chose not to because of how computationally excessive it could be.