I always got the impression in Tolkien that the orcs were not particularly threatening, in of themselves, but simply because they were united under one leader who wanted conquest - Sauron. Whereas the humans (and elves and dwarves) were not.
I think a good (if perhaps touchy) analogy is American Indians and Mesoamericans vs Westerners. Their technology was not nearly as high, and they weren't particularly united, preferring to fight with each other
I think the problem in D&D fantasy settings is that orcs aren't a threat, but other human nations aren't. They probably shouldn't cooperate so much. (But then again, gods and alignment would change things dramatically)
You miss a few points then.
1: Orcs are a mostly subterrene race. They occupy caves and dungeons below ground quite a bit.
2: In Greyhawk at least if I recall correctly there actually are some regions with orc populations. But keep in mind the whole area is in a state of re-population and migration into what was prior a mostly sparsely populated land I believe.
3: Why do they
need a Mordor? Bandits and other threats crop up and can be a major problem just the same.
4: This one is a little more obscure. But. Also potentially orcs or anyone else can have a Mordor. Just on a smaller scale. Due to the random habitation tables you can and eventually will end up with orcs and whatever occupying castles and ruins which gives them protentially dominion over the whole hex they occupy. Or not.
5: As someone else pointed out above. Technically there is a Mordor in Greyhawk. Iuz's kingdom.
6: In other settings it is all over the place with orcs all over. Or settings where orcs occupy a sizable kingdom even. In FR the orcs had a whole board game dedicated to their goofball kingdom wars. And in the Known World orcs and goblins controlled quite a bit of land. Especially goblins with no less than four regions.