Honestly (and I don't look at a lot of things in this way), "NPCs don't have to follow the same rules as PCs" feels like adversarial DMing to me. I've never understood why people see that approach as a feature rather than a bug. Part of what I want as a player is a consistent world where I can accomplish the same things as the characters I meet. I'm totally fine with there being believable requirements that the NPC met that a PC rarely would (like sacrificing infants and sanity to fuel some powerful dark ritual). I'm also okay with the DM not putting much thought into it, as long as they will come up with an answer when it becomes relevant. For example, it becomes relevant when I want to trick out my wizard's tower with the sorts of cool permanent effects NPCs put in theirs. A good way to deal with that, for me, would be to use whatever magic items creation rules are in effect. Or to say that learning how to do that sort of thing is the sort of lore wizards are always searching for, and I can accomplish the same stuff by seeking out such magical lore.
I'm not sure if the sentiment is just a conceptual backlash against the concrete way 3e NPC/monster rules could grind the game to a halt (believe me, I understand how much of a pain those statblocks could be), or some unconscious DM power trip, or just a bandwagon effect. In any event it completely rubs me the wrong way.
5e also walks a subtle enough line (at least in the earlier products, later ones have veered further away) that you can easily (as I do when DMing) say that they do follow the same basic rules, the stat blocks are just simplifications to expedite play. A veteran statblock represents what is really a 5th level fighter if it somehow became important, and most commoners don't actually have straight 10s for ability scores.