I'm kind of on the fence on this one. On one hand, yeah, it would be nice to go back and reexplore some of the areas missed.
On the other hand, it makes perfect sense why you cant. Breaking it down for a second, it is an RPG that heavily encourages exploration. Next, it's in the South Park universe. If the show is anything to go by, there's generally a huge mess, and then it gets put back to normal. I believe the same idea applies here in the game. While inconvenient, it makes sense.