Li Shenron
Legend
So how come even guys like Grazzt seem to have non magical weapons? I get that all their attacks are effectively magic, but almost no weapon wielding monsters seem to be able to find even a +1 weapon to use? You think being Demon lords or even just a high ranking Marilith they could swing something like that.
As the others say, I think it's because the game should work also with near-zero magic items, and adding something is generally easier than removing it.
This is not only a balance thing, it is also a gamestyle thing. There are always gaming groups wanting a low-magic campaign, and magic items scarcity is a major element of that, although there are many ways to intend what is "low-magic".
So if you e.g. set the initial situation as a world where magic cannot be made into permanent items, that applies to BBEG as well.
As for the balance, in theory the attunement rule should take care of the problem of amassing lots of magic items from killing monsters. However I do think that the attunement rules need some, well... attuning when you play low-magic. In a zero-magic-items campaign, attuning just doesn't come into play. In a lotta-magic-items campaign, it doesn't matter how many items to take from monsters, you're still limited by attunement. In a few-magic-items campaign however, you might want to decrease the max number of attunable items, otherwise after a while you still end up with PCs having the same amount of attuned items as in the lotta-magic-items case.
Also, truly important BBEG could use ad-hoc artifacts like the Wand of Orcus, and have their own rules which make attunement irrelevant.