It's a game. The fact that they use 5e for most things, doesn't mean they can't include other rules or explanations. A wish spell might not even exist in this universe, and it would be fine. Revivify scrolls don't exist and don't work this way in 5e. There are no "surfaces" in 5e like they exist in bg3. A lot of feats are changed (so far as to even break bounded accuracy). They could easily implement lvl20, but they decided to end the game around 12, which is fair. Let's not act as if they had no choice though