Lv 20 is obsolete. A party beats strong foes at lv 10-12. I had a 5 year long 3.5 campaign with nerfed xp gain(about 10-20% of what it should) and by the end we were saving the world at lv 12 and it was enough to face CR 16-20 shit regularly. I also played higher level short campaigns in 5e where cr 20 monsters regularly got wiped with ease by a moderately optimized 6 man table. A dragon didn't even damag the party cause my aasimar cleric/druid outhealed it's damage and undid it's CC nearly faster than it applied it. People don't like later levels cause things get abstract. Planar travel becomes common, teleportation is used in combat regularly(teleport in or our it's just an action). We had a PC die to a side-quest bbeg teleporting across the continent to gank us in a dungeon and we had losing fights get no sold by teleporting to a safe tower protected by the mother of all wards. Reality warping becomes common once wishes are in play, mind altering things, time magic, you name it. Things stop being conan thet barbarian or lodoss or slayers or lotr or whatever your dnd inspiration is. It starts being some Malazan shit where gods are actively involved and no-one has an idea of what magic fuckery who is doing to fuck with reality. Unless the DM just pretends actors who can spam wish, planar spells etc don't do it. Most sessions at lv 10-12 in 3.5 involved multiple planes and continent and at least some reality warping, races being wiped out in hours etc. It also involed some disparity. Optimzied wizards like mine or my friends cleric can do a lot more reality warping while an optimized martial is still hit with stick. Spell slots stop mattering when you got rings of wizardry with high 20s or 30s int like my wizard did. If the DM drains 80+ spell slots from one PC then the bad guys deserve to win.
It's the same reason most superhero stories don't have you throwing galaxies like Guren Lagann. Just cause it's stronger doesn't mean it's interesting or relatable. Most people like being around lv 3-8 in most dnd editions, they want to be upper street to city level adventurers engaging mostly with their immediate surroundings rather than fighting 27 dimensional god wars. I say this as someone who prob played 3-4k hours of dnd across the editions in the last decade+ and seen most of the patterns. Higher levels exist so the players know there's heights beyond them individually usually, the living gods and heroes of legend that honestly the party still beats if alone due to action economy usually.
layer to manual and you can control the whole party. Just have to change it for every fight but you can do it.
