I just assumed that orcs are too heavy to ride wolves, and they barbecue all the horses they find, so they delegate all the riding around to goblins and get on with what they best at - handing out brutal violence on foot. Dwarves also don’t have their own mounted troops, presumably because horses aren’t ideal in mountainous areas and mine ponies make poor cavalry mounts. Hiring human cavalry makes sense for a dwarf overlord.
I haven’t looked for a while, but I think one of the later Oathmark supplements allows orcs to take chariots of their own. Same for dwarves.
That’s not to say that adding a territory to orc and dwarf lists that allows taking their own mounted troops (and putting together a stat line for them) is a problem. The proposed orc cavalry don’t seem like they would break the game.