That's a fair point. Particularly in towns and cities. To expand on it further, could you imagine if you killed an enemy somewhere in the wilderness and came back a few weeks later (in game time) and there was evidence that the surrounding world had continued in your absence? Say if you researched the body/chest/whatever, loot that you didn't take from them had now been taken by others that have come along. Or the body was missing a limb as though a wild animal had been along.
No impact on gameplay itself, but all little touches that immerse you further into the games universe.
I get what you are saying, but at least for Fallout, I'd completely expect anything worthwhile to be scavenged by someone 12 hours later. Which would actually be awesome. I liked the Caravan thing in Fallout 3. Would be nice if all the NPC's just went about their own business in an open world game rather than being dropped in (sometimes literally) once you get near them. It would ideally help deal with economic issues as areas you visit pass your money or gear on through the game world.