Frank Trollman, Pun-Pun and plenty of others...
None of which exist or even could exist in my setting. Few games allow the full breadth of broken material necessary to pull off those sort of things.
...have pretty much debunked that type of thinking as short-sightedly unrealistic. In most non-D&D-derived fantasy fiction, any wizard above 6th-level is the in-universe equivalent of Superman. In most D&D fantasy campaign settings you have hundreds of 20th-level wizards (who are the equivalent of Doctor Fate, Doctor Strange, and Kriemhild Gretchen in terms of sheer world-altering/destroying power) running around one planet and yet the technology level and standard of living stays exactly the same for tens of thousands of years.
You keep spouting off generalities like that with no real facts to back them up.
My homebrew world of Korrel has so far as I know exactly 3 20th+ level wizards on the whole planet - the Lord of Dee, the Master of the Mystic Isle, and the Head of the Provost Council. The first one lives in a tower and no one has seen anything from him but his spells in 300 years, the second lives on an isle that is said to be intangible to all those he doesn't want is visitors and is surrounded by a perpetual magic fog and he's scarcely less often seen, and the third is a lich that never leaves the University catacombs and spends most of his time slowly decaying and torpid. My current campaign features a bad guy who is a 17th level wizard - Keeropus the Many Hearted. He's the first Archmage that's been seen in the east of Sartha (think Asia) in 80 years, since Halivar of the Many Colors died. In the nation of Amalteen that the PC's just left, the highest level character in the whole nation (100,000+ citizens) was 10th level. The nation they've just journeyed to, mighty Talernga, the Pearl of the Storm Coast, one of the greatest city states on the whole planet, after the bad guy, the two highest level characters are 14th level. One is a 70 year old human priestess of the God of Beauty who has no physical stat above 6 and ~30 hit points, and the other is a 700 year old Elven expert who is the kings' mayor of the palace.
We've been playing for almost 4 years IRL now, and the PC's are 6th level. During that time I've had 8 or 9 PC deaths. Only one PC has survived to this point from the original party of 6 1st level characters. The world is just not covered up with high level characters. In fact, in my game, the 6th level PC party constitutes high level characters, and none of them have the ability to craft anything fancier than a smoke grenade.
Fantasy campaign settings are kept in artificial, sanitized renaissance fair conditions, rather than becoming a magical version of Transhuman Space/Eclipse Phase/AT-43/Warhammer 40,000 on steroids, solely because of author fiat.
About 5000 years ago, in the age of the Art Mages, the Time of Wonders, there were hundreds of 20th level characters. It looked roughly like magic powered science fiction, I'm sure. The beginning of this campaign involved the bad guys accidently setting of a malfunctioning 'mass driver' and unleashing a tsunami that devastated half the country. Anyway, as often happens the humans decided they wanted to usurp the heavens (and the hells for that matter) from the Gods. The resulting war - the Iconoclasm - devastated Heaven and Earth (in the Hells, you couldn't really tell the difference). In a desperate bid to win the war, the leaders of the Art Mages create the Apocolapse Word of Power (Power Word: Planet Killer) and threatened to use it. The gods called their bluff; they weren't bluffing. To prevent them from speaking it on Korrel and destroying what the unknown creator had made (none of the gods have the power to make the world), at the moment the word was to be spoken the united gods ripped a continent off the planet and hurled it in to the Astral Plane.
Since that time, both the gods and the surviving mortals have a rather skeptical view of magic in the hands of mortals. In many parts of the world, sorcerers are killed as children. Those that survive are treated as monsters. Bards are little better off, but only because they've hidden themselves away in secret colleges so well that 'spell singing' is widely regarded by most right thinking wizards as being merely a myth. Shamans are generally burned as witches in most human lands and must survive on the sufferance of their local community. One bad crop, and they'll likely to find torches and pitchforks at their door. In all parts of the world, wizards must make their presence known to magistrates or face prosecution as witches. The clerics and champions of the gods vigorously seek out those that might be practicing the dark arts, and any mage that gets close to learning the secrets of art magic may well find the gods personally arranging his death. For this reason, most high level mages become strict recluses, figuring that as long as they don't intervene in mortals daily affairs, the gods will tolerate their personal quests.
All this happened because of author fiat, it's true. But don't tell me that Korrel maintains a fairly low tech level as a result of me handwaving away the implications of the setting.