DnD: Why is the other half of Half-Whatever always human?

It's an under-discussed fact (particularly among human cultures) but where other races have intrinsic qualities like luck or racial spells, humans have the magic ability (and the moral flexibility) to breed with nearly anyone. There are competing theories on why this is, and in fact among Sun Elf scholarship there's a whole anthropological subcategory devoted to the question of, basically, "Dear gods, WHY WOULD YOU DO THAT?" Is it a gift from whatever deity originally created them? A curse laid on them (so to speak) by some long-extinguished enemy species? Is there actually such a thing as truly purebred human?

Dwarven scholar Durdek Two-Hammer claims that the behavior comes from a fundamental instability in the human bloodline, and speculates that without breeding in bloodlines from other races humans would eventually become infertile and go extinct. The gnomish wizard Litha Glitterball, however, has published a number of scathing critiques of these claims, claiming that the sheer number of humans in the world could not possibly exist if that were true, and that Durdek should stick to the truly wild all-dwarf parties that earned him his name. Sir Artur Proofwaithe of the Human Anti-Defamation League (whose motto is "We Can't Be Racist, We Allow Half-Elves To Join") claims that the ability isn't magical at all, but instead reflects the "fact" that humans are the original bloodline that all other "humanoid" races were created from, which is why the human bloodline is compatible with all of them. (n.b. The Waterdhavian Scholarship Exchange does not hold with the use of this offensive term, and also notes the circular logic which undergirds it, as Proofwaithe elsewhere defines "humanoid" as "any race capable of breeding with humans", a category that apparently includes even dragons.)

Not everyone is troubled by this peculiar racial trait. Famous moon elf therapist Mona Highglow has long advocated for bringing in one or more humans to "spice up" elvish marriages which have fallen into a bit of a sexual rut after a century or three. More ominously, the infamous Grigor The Butcher directly attributes his success in looting, raiding, and pillaging to his mixed parentage and urges the orcish clans to take full advantage of the services (both military and sexual) of the more ambitious, mercenary, and horny sorts of humans. Halfling courtesan Delicia Gallops also famously announced that humans were "an experience everyone should have", a controversial remark that was widely associated with her rise to prominence in the Erotician court.

This proposal will explain why it is so vital to perform further research into this unique and under-studied phenomenon, and why the authors are uniquely situated to do so. It is our hope that the Whaterdhavian Scholarship Exchange will see fit to fund this important field work.