how enemies only appear at certian locations tes does a little to solve this with creatures ( wich i thought would have carried over to fo3) but nope just the same old place same old enemy in both games actually

Well, creatures and enemies don't respawn in enclosed areas, at all.

If you killed a Deathclaw in a sewer, and left for a month, then came back...it would still be there. Even outside, in some places, enemies will not respawn and there will not be that same enemy as before. In very open places, however...enemies do respawn(If they didn't, then you'd eventually have nothing to kill in the wasteland). For example, the raiders, outside of the Springvale School, or the Talon Company Mercs, just near the Riverboat Landing. They all respawn. And i'm sure a lot more do too.

I guess Bethesda just tries to make sense with what they do. If they had Raiders, outside the Springvale school one day, that you had already killed, and then respawn with Talon Company Mercs, with Raiders already inside the school, it just wouldn't make sense. That's why the same species/gang of enemies respawn, at their habitat/fortress that they belong to. It just makes more sense, than just jumbling up other types of people, into locations, that would be fit more for another type instead.

(E.g. Putting Talon Company Mercs, to guard the outside of a building, that doesn't belong to them, with other Mercs inside)

^You never see that, because "raiding"(loitering inside buildings, that are in the middle of nowhere) is not what Mercs do.

...

what i mean is why don't we ever see some random gang of raiders go out and explorer you know like a scavenge party? or kill a group of enemies one place wipe all them out then that area may get taken over by another group...

Because that was 2008 and made by Bethesda game studios and now is 2012.

I sure hope obsidian develops fo4...

New Vegas had this

You are joking, of course. New Vegas had a desperately static environment where you knew exactly what enemies you would find at any given location.

While particular locations such as, say, Bethesda Ruins, will spawn the same enemies over and over in FO3, the Capital Wasteland as a whole is a rather dynamic place. Every time I walk through it I see something I've not seen before, some odd combination of creature spawns doing battle.

Because enemies spawn where their bodies end up, not where they originate. Though they want to return to their original spot after about seven days. I found this out by abusing the system, that is, I used to set up massive Enclave vs. Raider battles at the Super Duper Mart.

That's true, but out in the wastes the creatures themselves spawn at random. One day you might run into a sentry bot rolling around, the next time you pass through there's a yao guai.

I have thought that it might have been good fun if killing all the 'static' spawns in a particular location resulted in a (random) change in faction. For example, if you kill all the raiders in Bethesda Ruins then the next time it respawns they are replaced with giant ants, say, or Outcasts.