The issue with [going from level 1 to 20] in a single campaign and so fast is that you basically upgrade gear all the time, which sounds fun but makes special loot less special.
This, but focusing more on the frequency of leveling up. If the game's length was doubled or we only controlled a single character, then sure we could expect to go to level 20. But as currently is, especially given that we have to level up 4-10 characters each time, leveling up almost twice as often would be way too frequent.
It wouldn't give players time to get used to their abilities, spells known, and strategies of what worked and didn't before they'd gain an entirely new set of spells. Plus, the enemies would also be increasing in power/complexity at this pace, so players wouldn't ever get used to typical enemy strengths/abilities/tactics.
Toward the end of Owlcat's games I began to dread leveling up (sometimes even not doing it) because it happened too frequently, I had to level up too many allies, and each level up (due to Pathfinder's extensive feat system) would add significant new choices & abilities. It began to feel more like a chore than an exciting event.
