The Crimson Binome
Hero
If that was the case - if the NPC wizard was just a PC wizard with a lot of the cruft removed - then that would be fine. It's not, though. The NPC has more HP and lower save DCs. It will survive to cast more spells, or to escape, which would not happen if you had chosen the other model. It could determine whether a PC lives or dies, or consumed a valuable potion which would otherwise be used to survive at a later date.[...]especially when considering that the practical difference is only that when a DM looks to see what an archmage NPC is capable of doing in the span of the 5-ish rounds that it will be in play, there aren't as many lines of text spent on information of no immediate relevance.
Monster-built NPCs are so far from PC-built NPCs as to be entirely irreconcilable. You say that you care about sharing a fiction with your players, but any fiction derived from such an inconsistent process would be terrible and full of holes.