Helldritch
Hero
Consistency. Predictability. Ease of use.Pre-emptive edition war aside, and editions aside entirely, why do YOU think that spellcasting in particular should be the same between PCs and NPCs and (spellcasting) monsters?
More than that?
Ok.
Availability of power to the players?
If an NPC spell caster can do it, so should the PC spell casters. A new spell? Ok. Can I make the research? The DM might put some restrictions, research costs and so on. But it would be possible. Not necessarily easy, but possible.
With at will, daily, x/day powers, it is simply not possible for a pc to achieve the same as a "playable" NPC. (See the Fizban Book and the NPC in there. No logic at all!)
Spell casting monsters have always used at will and x/day powers. Some of their powers can't be replicated by PCs and rightly so.
And for non casting NPC. Even their abilities can be replicated by martial classes. Especially the Battle Master. Rogue like NPC can be replicated by PC and so on.
Use the pregen then. Change a bit and voilà! It took you about 3 minutes of your time but your players can achieve the same power as that NPC. And if it is a wizard, they might even find the spellbook somewhere. A very prized item indeed. The problem in 1-3.xed was that there were no template. Not so with 5ed! We got the templates we wanted! It was a great win for everyone. Now, we lost it for no reason for the one you put above. Bravo...Casters are especially annoying to make like PCs. Picking 20 spells for a chump that's not going to last 6 rounds? No thank you.