Actually, I think I just changed my mind. Maybe it would happen in a real game, as part of a BBEG plot. A BBEG might actually be crazy enough to go without real sleep (only catnaps) for months on end, and to horde thousands of bodies, just so that he can unleash his Army of Darkness on his nemesis (the Free City of Pythium!) and have his moment of murderous triumph. Then he doesn't care if he can still keep control of the skeletons afterward--they're already in the city.
The PCs may get hints of his ambitions early on, and may have a chance to stop him (epic fight against a guy with nigh-unlimited Armor of Agathys/Dimension Door/etc., and probably a dozen or so skeleton bodyguards at all times even while he's storing up spell slots--presumably he just casts Animate Dead III out of warlock slots) but there is also plenty of room there to explain why the guy is passive enough to give the PCs a chance to stop him: he doesn't want to go out on missions to find out who is destroying his armor and weapons caches and corpse stockpiles, because he's busy resting.
Honestly, that idea sounds kind of awesome to me. It's a little bit like Brandon Sanderson's Reckoners series with the villain Obliteration: he lies around in the sun for weeks on end, storing up energy, and then he destroys a city with the stored-up energy. Same vibe from this necromancer chap.