Say you are a clever DM. You discover that nearly any mage (or bard) gets Clone spell: not only do they re-gain their youth with each casting but now they can carefully place at least one (if not thousands) of long-term insurance-policy bodies. Such casters are the classical definition of 'immortal': they neither die from time nor violence. This immortality can possibly be attained with Reincarnation for the more druidic sorts. In all honestly: all one needs to build a low-tech immortal is to make a copy of anybody's body and have it petrified until needed. Can one put Break Enchantment on a trap? If so, slaughtering any hero with a back-up body accomplishes nothing but developing immortal enmity.
Now you have a problem: any NPC with intelligence &/or wisdom score higher than you (the DM) has already thought of this. If any NPC knows their basic spell lore they will find someone who can make them spare bodies. Thus, if you are that caster yourself you develop armies of immortal superheroes (that depend on this body-making caster for eternity). Now, if you assume that this logic has persisted for as long as D&D magic has existed - you suddenly have thousands if not millions of super-powerful NPCs that cannot be killed. How do you gently fit this metaphysical concern into your campaign without throwing the entire thing into a cheesy mess?
This sounds like a bizarre solution, but consider the evidence. Imagine someone could have everything they ever wanted. Everything! Actually, beyond everything. Imagine beyond having all the money. Heck, you could make money. In truth, this caster could make anything that money could buy - or infinitely better than anything money could buy. With that kind of product production, why would you even bother going to town at all? This is the logic that saves you, right here: once any mega-caster can develop the best products 'in house' they will simply never leave home.
Let us look at the philosophy of great psychological theorists down through history and see if this is the case:
This one is the easiest to imagine: consider the use of True Polymorph. A caster can make virtually any blob of matter into the girl / guy of your dreams. Poof, there you go… no need for Tinder. This date lasts for an hour. Was that fun? If so, cast it again for another hour. Ready for commitment? Concentrate, as the spell requires, for an hour. Find this relationship isn't working out? Teleport yourself (and everything you want to keep) to another plane of existence / don't leave a forwarding E-mail address.
The clever DM will swiftly realize: if you can make any body you want for yourself you can easily do so for whatever you enjoy playing with. You never have to be alone ever again… unless you want to be alone (which is where most mages end up with - more on that below).
This one is a bit more tricky to explain why this won't ruin your world. After all: casting True Polymorph every day gives you three hundred and sixty five sentient (and rather powerful) creatures per year. As these creatures can be anything up to CR9, you can make some very powerful servants / armies indeed. Flesh & Clay golems qualify under RAW - you could also allow weaker versions (less hit points / smaller) of stone & iron golems as well. Like the Shield Guardian? You could make powerful versions of any Animated Object you like, perhaps building the entire cast of Beauty And The Beast - including Gaston! Make one dragon a day - as time goes on they will get older and up to incredible power (remember: you have forever to do this). Assuming you only make lawful creatures and you treat them well (why not, you can make anything at this point), you now have an infinite and all powerful army. We won't even go into what you can build on timeless mini-planes - thereby rendering your creatures immortal. Imagine: you never have to mess with the dirty, ugly & stupid undead armies ever again. Just super-powerful creatures doing your will, transforming the world into whatever you want. Right?
Not so. As you are ultra-smartwise you have figured out that the world is filled with people. These things are not so smart. Nor kind. Nor much fun either. In fact, they are kind of revolting. More than just revolting: they typically revolt over the most petty and trivial thing you can imagine. After watching them for thousands of years with your level of genius you begin to realize that these common folk (even their leaders) are the most disgusting and unsettling things to ever be invented. Arch mage says: "Arch Duke Ferdinand… assassinated? Here I thought Ferdinand was a bull."
So mages tend to keep to themselves. A lot. They may have entire realms of toys (for a few thousand years) until they bore of them. But fixing the world? Yuk. They made this mess for themselves - perhaps they should get themselves out of it.
Over thousands of years with vast and renewable power, most high-casters can play out their entire story. This is ultimately what happened with Vecna. By first edition he was already an immortal lich. Somewhere around 3rd edition he became a 'god'. What happened since? Now he has been searching the vast multiverse for 'knowledge' or something (and not much has been heard of him since). Sure you get the occasional cultist but they don't seem to know what they are doing nor why they are doing it. What does a Vecna-cultist even say as a battle cry? 'May you read interesting… but bad… stuff.' or 'Death to everyone, um, except undead as they are fine as they are.' Compared to those serving someone like the God of Justice these guys seem rather dull.
This is the ultimate draw as a sentient caster: they discover they can play out their entire life, meaning, story &/or *raison d'être * by not even touching the status quo of normal folk on the Prime Material. This withdrawal is further reinforced by the fact that the impoverished normal folk want to take whatever these casters make either by demand, trick or force.
What is interesting for many a DM: most wizards may be any kind of psychological deviant. Possibly ADHD like Mr. Walt Disney or Dr. Albert Einstein or any kind of autism-spectrum re-orders such as Aspergers. Genius, yes… but not specifically social, possibly even introverted. Such people want to enjoy their imagination and possibly little else. Attempts made by these introverted casters in sharing this brilliance with the world could go badly in an Edward Scissorhands kind of way.
Skinner suggested we are just machines reacting to various stimuli. Given access to the very best stimuli one would assume many mages and even bards may get glued to their creations not unlike the internet-experience we have today. Dr. Carl Rogers suggested that all we really want is human connection and/or the approval from a few good friends. Why would anyone want any more than that? Who wants too many friends? Who wants contact with all that apathy that exists in the world anyway?
Suggested conclusion: It is possible that thousands of years of Arch-mages, Grand druids and even College-Dean bards eventually cause themselves to be distant to the point of irrelevant - they may be 'all powerful' but this power matters to no one, not even themselves.
As a DM you must be careful about the actual discovery of a buried and long-forgotten caster. This was what the Tomb of Horrors was about. Gary Gygax figured out that there is very little left of Acererack. He is nothing but a skull and some dust that never moves for centuries as his spirit wanders the multiverse. There is no loot except for curses, traps and misery. It may well be the ultimate evil, sure, but he basically wants to be left alone. Sure it was the harshest dungeon ever written and many characters have died there - but what is the point? Actually, why go down there at all? Never met a player nor DM that could answer this simple question.
But this is D&D after all! Here are some events that may make use of a high-caster:
That's plenty for now. Thanks for reading.