Well it doesn't exactly say anywhere it's supposed to be a barren wasteland. The valuable artifacts and the stories about the wish granter attract the stalkers. The monolith stalkers also have their own reasons for being there as you sort of find out towards the end, as do Duty and Freedom in there own way.
I do agree about the spawning though, it's by far the worst thing about the game. It happens everywhere too. The tunnels under the research lab, I came out of there having cleared them completely, went straight back in since I'd come out of the wrong end (long story but I also went in the wrong end, doing the tunnels back to front, so the mission status didn't update when I came out the wrong end) only to find it suddenly full of enemies again
![Roll Eyes :rolleyes: :rolleyes:](/styles/default/xenforo/vbSmilies/Normal/rolleyes.gif)
There needs to be some spawning of course, but it happens far too quick. You end up just running past spawned enemies since you get fed up of killing them.