I don't see a reason why NPC's must follow the same spellcasting rules as a player.
Consistency, for one?
It's far more enjoyable to know that all the pawns on a board are playing by the same exact ruleset.
it's the same reason a lot of people absolutely despise when the AI in Civilization cheats and creates new units out of thin air, regardless of how difficult is the game.
but at the same time we have creatures with Innate Spellcasting, and some (like drow) that have spellslots and Innate spellcasting at the same time, allowing to cast way more spells than a player can. Plus, although they are not present in BG3 there are feats and class abilities in 5e that allow players to do similar thing (like the Tasha's Ranger, Fey Touched feat, etc.), albeit at a much more limited scope.
But these are ALL implementations of the rules, not arbitrary exceptions to them.