It feels very contrived and is extremely poor design.
I don't understand how it is poor design.
It is quite acceptable for a NPC to have wealth that a PC of that same level/mechanical capability is not entitled to have. For instance, there is nothing wrong with statting up a princess who is (say) 2nd level and yet has wealth (or magic items, or servitors, or whatever) comparable to a name-level PC.
So why is it problematic to give that princess some other capability that is off-limits to PCs? In both cases, the reason it's off-limits isn't because in the fiction it would make no sense for such a person to be adventuring, but because it would upset game balance.
It's possible to design all your NPCs (and antagonists more generally) in such a way that they satisfy the balance requirements for PCs, but that is an extremely significant constraint on NPC and monster design. (3E doesn't fully aspire to this, but leans the furthest in this direction of any edition of D&D; HARP - a Rolemaster variant - has similar aspirations; in both cases it causes needless distortion in the design of monsters - eg hippos either end up having too few hit points, ludicrous CON scores, or too many skills).
by the time 3E came around it was probably assumed that most evil humanoids would eventually need to be playable as PCs, so they just avoided handing out abilities that would be broken for the PCs to have.
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They never entertained the idea that it would be okay for an NPC to have a spell or ability, but that the same individual would lack that ability merely by virtue of becoming a PC. That would be ridiculous.
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And so you end up with shiny abilities at each level, and at-will magic, and fast-healing; heedless of the costs associated with these, in making NPCs too complicated to play
Just as I don't see why it is inherently superior design to constrain NPC and monster building by reference to what makes for a balanced PC build, so I don't see why the design and build of PCs should be constrained by considerations of what makes for an easily playable NPC.
GMing and playing are different game functions. They don't need to use the same tools.
If you expect the DM not to think of the game as a game, I'm not sure how that would work.
For a start, you might get NPC and PC build rules treated as ends in themselves, rather than means to the end of playing a game!