That's exactly the sort of answer that if I were a player would completely put me off.
The problem there is that a player never complains that, as a PC, he has powerful abilities that most NPCs will never have. So why should he complain that some NPCs have abilities that he will never have ? Why does he have a gift for magic or fighting or whatever that will make him a hero, and what gives him the right to think that he can have everything that happens in a vast fantasy world ?
By the way, it's fairly absurd to complain, because for example a fighter will never be able to cast spell like the wizard of the party. So why would he complain if an NPC can cast spell that he can't ? And the same thing for the wizard, he will never be able to cast the cleric spells. So why would he be able to cast all the spells that an NPC can ?
The world is not defined uniquely in PC-available axes of power, why should it ? Look at Lord of the Rings, Sauron has capabilities that the PCs will never have. Voldemort has access to powers that the PCs will never have. It's a core of the genre, that the PC with limited capabilities is still able to vanquish foes stronger than him and using forbidden powers.
If the NPC has a magic item that lets it shoot one fireball each day then fine; that's an item that is now known to exist in the setting and that in theory I-as-PC can get my hands on as well. And if I-as-character don't specifically know about the NPC's item I should be able to assume it or figure it out, as unless this "warrior" is also a significant magic-user it shouldn't otherwise be able to cast a fireball. It's called internal consistency.
The genre is full of surprises like this. The Death Knight (my players fought one Friday evening, eventually running from him after turning him for a short while) has always been able to cast a Fireball, ever since AD&D FF. So it's now Hellfire and it's even worse than a fireball, actually, but he can do that, always could, while there is no special reason for it other than he is an infernal knight.
And it is not inconsistent, it's just a very, very magical world full of possibilities, some were available to the players because of the path they took, others not, exactly like in every single instance of book/movie/show of the genre. The adventures are already, by the definition of the game, player centric.
So maybe the item above has been tempered using the NPC's blood, so that it can only work for him. Maybe it was a gift from his god/patron, and only that god/patron can create it because it's his portfolio, and he restricts it. All of that is part of the genre, but honestly I have no forbearance for players who have characters already very powerful and with abilities that incredibly surpass what most of the world offers and still whine because the boss has, in turn, abilities that make him powerful too, although in a different way. The PCs have their stories, the NPCs have their, and it's actually way more interesting that way.
And it also prevents metagaming what an NPC could possibly do. It might work sometimes (he's known to be a wizards and have studied there, etc.), and sometimes it won't (e.g. Voldemort above, or actually most evil wizards of the genre).
But just saying "because he's an NPC" as the rationale? Hard pass.
It's just because the other guy is an individual, possibly more gifted than the PC in other domains, and magic and power is not uniformly canalized along the path of the PC classes. It's a wider, richer, more vibrant world than that and there are many, many paths to power.