NPCs don't follow the same rules as PCs, never have in 5e, and that's ok.

[Psyren]

Then why are the thematics of the fighter NPC with at-will fireball a problem? With some number crunching they can be balanced. They're not PCs. They'd be designed by the same process for a single purpose, to be an easy-to-run 3-5 round challenge to a party of players whose level corresponds to the desired encounter difficulty.

All three kinds of wizard (PHB, VGtM, MotM) are capable of spontaneously producing external magical energy. Without a highly specialized subclass, fighters are not, and even with it their ability to do so is extremely limited.

Could you explain a rectally sourcing at-will fireball fighter in some way? I'm sure you could. WotC however has no reason to, and neither do I. Nothing is stopping you from doing so.