Yes, all the companions we meet should be level 10. But does the game need it? Is there a point in screwing the game mechanics derived from D&D just to satisfy the thrive for authenticity and logic of some players?
Again, it's all about consistency.
Maybe you don't really care... And I have to admit that in a game like DoS it's not a problem to me... But BG3 is not sell as a DoS game.