That was one of those things that kind of bugs me about so many other hominid species in DnD and the like. You're telling me there are this many species that are basically a human+X, but they're all distinct and separate? How does that happen? Even with racism, you still get the horrors of sexual violence that will cross those taboos. We have half-elves, but not half-genasi? And all the half-elves are half elf, half human, not half something else? Same with the half-orcs? Same with tieflings, etc? If there are no actual physical blockages to human/elf pairings, and half-elves are so much longer lived while still reaching adulthood in the same number of years as a human, how many generations do you really think you can maintain a distinction between them? Give it a few hundred years and you won't have elves and humans and half-elves, you'll have a complex gradient of very elf to very human to very orcish to very dwarven to every every other 'race' on the setting. At a certain point races in DnD make even less sense than the imaginary 'races' of humans in the real world.