Consistency, for one?
It's far more enjoyable to know that all the pawns on a board are playing by the same exact ruleset.
What consistency are we talking about? NPC's, like many creatures, have special abilities than a player does not have. Some have pack tactics (thug), some have extra damage (gladiator) etc. Are those abilities similar to what some races/classes have - sure. But by extension why can't "archdruid" have the abilities that Halsin has.
That said, even the "archdruid" from Volo's Guide (CR 12) is weaker than what a PC archdruid (lvl20 druid) is - both in stats, health, abilities, with only spells and wild shapes being somewhat equal. They are both called "archdruids" but are fairly different beasts.
So I really don't get why NPC's in BG3 need to play by the same rules players are, if there is plenty 5e published material that says otherwise. Especially when we talk about a named NPC.
But these are ALL implementations of the rules, not arbitrary exceptions to them.
Sure, there is a difference on paper between an NPC having 7 first level spell slots and an NPC having 4 first level spell slots and 3 free castings of "entangle". But when fighting against them that will pretty much feel the same. That said, combat generally doesn't take enough rounds for the NPC caster to use all of its spells anyway.