if they care about RP they do, if not it's just hack-n-slash game so it really does not matter who is killing who, just as long battles are mechanically interesting.
OK, sure. But how many GMs are actually going to come up with that history?
The post I replied to said something like "is it the orcs' home, or did they take over the dwarfs' home?"
Well, what if it was an abandoned dwarf fortress
(I should start playing that again...) because all the mines tapped out and the orcs moved in?
What if the dwarfs started a war against orcs because the orcs were sitting on a bit of land that was rich with ore that the orcs didn't care about mining, but the orcs won and took over the fortress (since killing people and taking their stuff is perfectly fine)?
What if the orcs
did invade and murder all the dwarfs, but that was
many generations ago and the orcs who live there now are innocent of that crime?
At some point, people are going to say "hey, that's too much work," and unless your an inveterate worldbuilder, it probably is, especially if you just want to go forth and kill some monsters. Which brings us back to... why is it OK to go and invade an orcs' fortress and kill them all?