Orcs and humans aren’t descendants from a common ancestor though, nor are they believed or presumed to belong to the same stock. Orcs were created by Gruumsh, and... well, the origin of humans is left unclear, but they sure aren’t believed to have been created by Gruumsh.
We are actually agreeing here; the definition of race is that you must have a common ancestor or belong to the same stock. Orcs and humans do not, so they are not the same "race." This same standard does not apply to "species" meaning all you need to do is be able to procreate.
Again, I don’t think we can apply the same taxonomic standards to a world where creatures are directly created by gods rather than emerging through evolution by natural selection. Orcs and humans might be able to produce viable offspring, but so can dragons and humans, and I don’t think anyone would try to argue they’re the same species, so clearly breeding compatibility is not a reliable indicator of species in this setting. The fact of the matter is, there is no shared lineage between orcs and humans, so calling them the same species doesn’t make sense.
A geneticist would likely disagree with this. If an alien came to earth, and had a baby that could in-turn have more babies, it would actually be evidence that the humans and the alien are the same species, regardless of their origin.
Consider grizzly bears and polar bears; for some time, they were considered separate species, because they don't live in the same area (they diverged 400,000 years ago), and therefore cannot procreate. It actually had little to do with common ancestry at all, as their both part of the same family (ursus) but their different habitats meant they weren't the same species. However, now polar bears are moving farther south and are mating successfully with grizzlies, (creating Pizzlies!) there is some debate whether grizzlies and polar bears should be reclassified as the same species.
Anyway, bottom line is ancestry is not as important to species classification as reproduction is.
I do agree with you that the modern application of "species" shouldn't really be used in fantasy, as most people in a fantasy world have no idea how genetics work. But if you were to apply them, orcs and humans would be the classified as different subspecies of the same species, which would have some silly name like "homo-humanoid."