For reference, the tiers of play in 5e.
1st—Levels 1 to 4: Local Heroes.
2nd—5 to 10: Heroes of the Realm.
3rd—11 to 17: Masters of the Realm.
4th—17 to 20: Masters of the World.
Level 11+ already means a legendary hero or villain, king, master wizard etc. For the setting to make sense, you can't have levels 11+ parties running around everywhere. They start to require very specific content because they are recognized as Masters of the Realm.
And Larian has to include sensible ways to deal with high level players who decide to attack everyone in Baldur's Gate or a similar location. Simply making all guardsmen suddenly level 9+ isn't a sensible solution.
