Why do GMs always want to restrict things to Core Rulebooks when they want to limit power?

More specifically for games like Lancer, DnD, or other games where balance matters and there's continual released content

It's understandable if they don't want complicate things but trying to make it more balanced? That doesn't make sense if you understand how the meta is like. In Lancer,a kitted up Everest with Assault Rifles was one the best build by a lot. A corebook only Cleric/Druid is going to be much more troublesome than any Dread Necromancer. Even In PF2, a bard has a much higher powerlevel than a Psychic or Witch.

The best reason that I could muster is that aside from lack of knowledge, most of the GMs are actually small-c conservative regarding the aesthetic of those games--Sure the Wizard is unbalancing the game but it's less.... well, 'cringe' than the Artificier