I think the in-world explanation is that timefall and voidouts have wreaked such havoc on the environment that the United States' terrain and vegetation is completely unrecognizable.

Ah. So time fall accelerates continental drift.

Ya know, that’s real hard to say.
I know you’re joking, but with amped up erosion, it stands to reason that timefall really would affect terrain itself post-DS.
I don’t know how much it would affect continental drift itself, since that’s an underground thing, and timefall is localized, and it becomes harmless water after it strikes any surface, so it can’t affect fault lines directly. That said, we didn’t see much of the world during the first DS, so it’s entirely possible that the Voidouts caused by millions (billions?) of deaths might have had some serious tectonic effects on the world (was there a superstorm of timefall when the BTs crossed over?), especially considering that the central US was essentially ripped in half, with a sea pouring into the crater....

If nothing else, it explains all of the dang rocks scattered everywhere...