There's a difference in tidying up and living in a clean home.
If my house would have a bunch of dead 2 headed cows, about 24 cans and a lot of rubble next to it, I'd sweep that stuff up or away. I wouldn't dust, but still keep my tables empty. No need to let 25 different empty cans pile up, not even I'm that filthy. (23 cans okay, but 25? come on?!)
Mankind can only live so many years in filth before humanity starts to get annoyed about something, and when that happens, it'll rebuild the world. clean up, project purity, etc.
it won't go entirely to normal, but at least the world will stop looking like a big rubbish dump. Perhaps the mutants will get rights/be removed, cities rebuilt, so the rubbish won't last anyway.
So yeah, it's good for the atmosphere, and setting of a game, but in real life people'd cleaned up earlier, planted trees and stuff.