It's because the system falls apart after lvl 10ish.
5E falls apart past lv 10. This doesn't means that other editions has the same problem.
but still basically unplayable and a nightmare to DM. There is a very good reason there are no high level pre-written modules.
The numbers are too big and I agree that it makes hard for DM's that aren't good with math.
ng .Then mechanically its not so good , past level 10 characters were quite overpowered, then you have to rethink the environments, you can fly teleport, planeshift, pass through wall, wish , the campaign get a to a bigger scope, political struggles etc..Those are ok on tabletop but doesnt fit very well in a CRPG , unless you go on a pure dungeon crawl. Balancing all that stuff requires experience too the 5E cr is botched.
This on 5E. On 1/2E magic users needs scrolls to learn spells, they don't learn spells from nothing, so if you don't wanna characters with fly, teleport, planeshift, wish, etc; you can ....
NOT GIVE THIS SCROLLS. And note that everything that the PC can cast, a enemy can cast.
Strahd on 2E had access to spells like Contingency, antimagic shell, control weather, death spell, guards and wards, teleport, wall of stone, finger of death, force cage, limited wish, spell turning(...) and it was able to tear apart a lv 15 party. They used teleport to flee? Just dimension lock before battle or teleport and follow then. They tried to nuke Strahd you with minions? Death spell. They tried to bombard Strahd with spells? Spell turning. Why even engage on battle, send a clone to deal with then and to dominate one party member. And this only looking to his spells. He has a lot of other nasty abilities and if the party is dumb, he can kill everyone in one or two rounds.
I'm not sure many people would agree PF:KM has good high level combat/gameplay.
What about Knights of the Chalice 1 and Knights of the Chalice 2?
What do your character's backstory/motivation/personality matter when your party are on their way to killing their 4th god?
You don't kill Gods on high level campaigns. On 2E, at best you can kill a deity's AVATAR which has a tiny fraction of the true deity power and is like lv 50+. A demon lord or a archdevil would be the last enemy which you fight in a epic level campaign. Is the case of hotu. You don't kill all archdevils, you kill just a single one and getting his true name is a long quest. Other high level "adventures" are more about civilization building like
Test of the Warlords(lv 15+) than about exploring dungeons.