They're also not NPCs in the sense that they are races that are playable as PCs. So that's not really a particularly useful direction to take this discussion. They're not a human necromancer with magical powers that aren't attainable by a PC.
I don't need an NPC's powers to be attainable by PCs in general. But I can certainly see the argument that anything a normal human (or elf, dwarf, halfling, etc) achieves as an NPC as far as amassing weird powers should be something a PC could do if they followed the same path. But, with the way things are in 5e, they aren't and are that way for purely game mechanistic reasons to provide a particular level of challenge and that frustrates some members of the D&D community. I get it. I don't need it. But I'm also not going to put it down. It's just a different preference. There's no real point in arguing against it other than to recognize that this is the way 5e is because they're emphasizing the game aspect of the role playing game with it. And in doing so, it does narrow some of the role playing paths that a PC could take because they can't say "I want to do be able to do what HE does" and achieve it.
Everything comes with tradeoffs.