Wizards, that's wizard with a capital W, tend to be egomaniacs. It comes with the territory of knowing esoteric secrets to bend the universe to your will. They tend to get big ideas, big appetites.
Presume for a moment, that there is an ambitious wizard. A dark lord if you will. They've set up their fortress, mustered armies, and brewed up monsters to serve them. Pretty soon their expanding their personal fiefdom and waging war on the world at large, or at least those bits closest to them. If they're a particularly smart wizard they might even try to play in the local political playground of shifting allegiances and motives.
Eventually though, more often than not, the wizard unsettles the existing power structure enough that someone, or an alliance of someones come along and kick their door in, blows up their fortress and scatters their armies and creations to the four corners.
The wizard likely dies and the remnants of their little empire become brigands and wandering monsters that plague the countryside, if they aren't recruited into the retinues of winning side that is.
In the war's wake veterans of the winning side might well become monster-hunters or mercenaries or brigands themselves and form a class of wandering rogues, neer-do-wells, and problem solvers. Adventurers.
Meanwhile the dark fortress lies in ruin, but a ruin still packed with the accrued wealth, occult artifacts, and other seductive treasures that the winning armies missed during the sack. Inevitably such plunder lures your usual miscellany of scoundrels. Perhaps one or two are magic users themselves and they dig up arcane secrets out of ruins, ally with remnants of the old wizard's armies, and starts the whole process over again.
Now iterate, repeat this process for a couple hundred years till the ruins of wizard fortresses and holdings litter the landscape. Every time an upstart new witch king is smacked down you have shattered armies bumping around and suddenly a market flooded with arcane artifacts and treasures.
What about the previously mentioned victors, those powers that be?
More likely than not they're simply the descendants of those wizards that managed to actually succeed in carving themselves out a kingdom and sustaining a lineage. The truly effective ones are the ones who incorporate the creations of other defeated wizards into their ranks.
In fact its likely all the world is ruled by wizards, to one degree or another, at the very least anyone in power is keenly interested in the creations of wizards. Feuding, fighting wizards who have big egos and a tendency to try and build their own little empires on the ruins of generations of other feuding wizard rulers.
It is an inherently adventure-some world, the constant cycle of wizards making monsters (an entire world built on "a wizard did it") and fighting with each other both produces ruins to loot and class of itinerant adventurers to hunt that treasure and deal with the general resulting mess.
It supports both gonzo and pulpiness, everyone is building armies out of the defeated troops of other wizards and steal each others magical secrets and arcane technologies. Dials can be turned for each element. Want more ancient ruins of past civilizations? No problem, sprinkle in some of those. So many pulp ancient civilizations are sorcery themed anyway.
Such a setting provides the "aesthetics of ruin" but with a route potentially circumventing the typical fallen empires.