Breathe some spirit in the form of Mudzombies? Eh??
Like Meech kind of said the Vaults felt like dead, ancient places. Whilst the natural environments were grand and linked Abe to his spiritual ancestry the native areas in AE seemed incredibly hostile and grim in comparison.
Although I don't think this is a bad thing. It's a way to distinguish them from the Monsaic/Paramonia/Scrabania areas since they're essentially the exact same thing.