MechaTarrasque
Hero
From a game mechanic perspective, like everyone else said, balance.
Once Upon a Time, though, demon lords were a step below demi-gods, with cults and worshippers and hell, those cultists had spellpower. Those demon lords were too badass for mortal trinkets! Sure, a weakling like Thor might need Mjolnir because he's too unimaginative to grow himself some tentacles or claws or the like. But practically, if you start carrying around artifacts instead of just zapping mortals dead with a thought, then mortals are going to want someone to stat up these god-like figures so they can fight them and take their stuff.
So Once Upon Another Time, designers caved to these bloodthirsty players and let them fight gods and take their stuff. Thor got 160 hit points and a 1d8+5 hammer that no one could lift. Then players could brag they took down a god, that Thor begged for mercy before they bitch-slapped him and made him drink from the horse trough. It was a rough time.
Once Upon a Very Recent Time,, designers thought it'd be a good idea to keep dumbing down these near-demigods to dough-like wimps who get whipped by a bunch of 10th level 20-year olds who just started adventuring 2 months ago and had good luck in taking out a goblin chief and his ogre. Nothing more embarrassing than being an eons-old near-divine entity of pure power who has ruled an entire layer of the Abyss for thousands of years by facing down countless droves of balor and pit fiend challengers only to get your butt handed to you by Rick the 22 year old fighter with +1 sword and Sally, who has been studying magic for approximately 6 mortal years. Six. And you get a little pissed off at Sally because she still knows more spells than you. In prior editions, you recall having a lot more oomph.
In summary, demon lords shouldn't need magic items. Maybe the Wand of Orcus artifact, if they had left it alone (originally 50% chance to annihilate anything it touches except other demon lords and gods). But the designers caved, gave us wimpy demon lords that leave you wondering why they wouldn't tote around some magic gear to prop up their sagging arses.
In the old days, demon lords would be handing out magic items like candy to tempt mortals. They wouldn't need a hoard as if they were some kind of mid-level dungeon boss. They're out taking over the demi-planes and corrupting souls, and they don't need trinkets or gold piles.
So that's why demon lords don't tote around magic items.
Me thinks there is something too this. Maybe Grazzt gave away his magic sword to your buddy the bladelock....so that means the bladelock's sword is your loot
