This isn’t true - even games like Inquisitor Martyr does seasons and I’m pretty sure like 99% of people play single player in that game (if anyone even plays it).
The point of a season is because in an ARPG once you’re geared and max level there’s really nothing left to do. The fun part of the game is supposed to be loot hunting, itemization, and character building (which I’ll admit D4 has some problems with).
In seasons you start fresh so you can play what is supposed to be the fun part of the game again.
To me the only reasons seasons in D4 don’t make sense is due to all the MMO mechanics no one asked for shoehorned into the game for no reason (well, so they can push MTX cosmetics let’s be real).
Take out the renown grind and a new character in this game would go pretty quick.