The vast majority of campaigns never reach 20. It isn't really necessary to have an enjoyable adventure.
Plus once you get to levels 15+ casters start getting pretty insane. A 20th level druid can Shapechange to become a full adult dragon while maintaining their druid levels and spells, as an example. And that is without touching on more crazy spells like Wish. Levels 15+ are in such an incredibly different realm of power that it might not be feasible to do in a small scale tactical RPG like BG3.
