I'm slightly inclined to disagree here. While their origins are pretty much nothing like our real life species they're still very obviously beholden to many of the same kinds of struggles; hunger disease fatigue, etc.
Hunger and fatigue would have nothing to do with reproduction, and there's no lore examples of a disease causing infertility or anything similar.
Humans, Kul Tirans, dwarves, Dark Irons, gnomes, orcs, mag'har and, to an extent, goblins all come from Titanic creations or were altered specifically by the Titans or their creations.
Night elves, blood elves, void elves, trolls, Zandalari and nightborne all come from the troll lineage.
Draenei and Lightforged have unknown origins, but are confirmed to be able to interbreed with humans and orcs, creatures of two different planets.
Tauren and Highmountains likely evolved from Niuzao like many other anthropomorphic creatures and their corresponding Wild Gods, such as harpies from Aviana, furbolg from Ursoc and quilboar from Agamaggan.
Undead can't procreate so this is a non-issue.
If some of those
couldn't interbreed with one another, I'd argue that'd make less sense than if they could.
I'd prefer something more than 'lol magic' as an explanation,
I think most of us would too, but "lol magic" is what Blizzard has handed us, and as the word of god says, the word of god holds. Any interbreed combination is possible with magic.