I don't understand this line of thinking. "Pick a parent" only applies to mechanical abilities. It's not like any of the half races had unique abilities to those half races. A half elf doesn't have anything that an elf or a human doesn't have. So, your half-elf uses elf powers and my half elf uses human powers.
Do people actually define the race of their character by the abilities of that race? Is that what it means to be an elf? +1 to hit with a longsword and a spell?
Half-Elf has Fey Ancestry - Elf has that, Human does not.
Half-Elf has floating skill proficiencies - Human has something like that (at least in the playtest), Elf does not.
Under "Pick a Parent", I can't have both Fey Ancestry and floating skill proficiencies. I can't have a mixed-ancestry character that feels like they draw something distinct from both of their parent lineages in any mechanical sense. That may not be important to everyone, but it is to me.
I don't create characters concepts thinking first and foremost about how I am going to get X or Y mechanic into my build - I think of their story, who they are, where they came from, and choices of class and ancestry are generally derived organically from that. But once I come to the conclusion that "This character is a Human/Elf/Khoravar/Dwarf/etc.", then I want them to feel that way.
Saying "Use an Elf statblock and call it a Half-elf" feels like saying "Play an Aarakocra and call it an Air Genasi" - you could do that, and maybe that would work fine for you, but if I had wanted to play a bird-man instead of an air elemental genie-man, I'd have chosen to play a bird-man in the first place. If I wanted to play an Elf, I'd have chosen to play an Elf - it's not about "+1 with a longsword and a spell", it's about the fact that locking my Khoravar out of half of their lineage for the privilege of choosing between Trance
or a Human bonus feat when I didn't want either one in the first place doesn't make them feel like Khoravar, it makes them feel like an Elf
or a Human, regardless of what I prefer to call them.
This is exacerbated when it's scaled up to world-building levels. Under "Pick a Parent", it means that on a fundamental level, the only shared traits that a society of mixed human/elves (or any other combination) possess are the aesthetic ones. If 50% of a society made up exclusively of half-human half-elves experience something as core to the "human" experience as
sleep in a profoundly different way than the other 50% of their kin, then they stop feeling like a single cohesive "people" and start feeling like two different peoples that happen to living in close proximity. It changes the dynamic and makes it feel like there is some kind of stark divide between the "human" half-elves and the "elven" half-elves that cannot be bridged, to the point where them even being "half-elves" in the first place stops feeling relevant - they may as well have just been humans and elves from the beginning, because at least then they would no longer feel like there's some fundamental level on which they ought to be able to relate to one another but ultimately just can't, even if they happen to be identical twin siblings that have never spent a day apart.
I have no issue with someone playing a half-elf that uses the elven statblock as a means of emphasizing a particularly strong connection to their elven heritage, but doing so at the cost of more fully "mixed" statblocks only serves to forcibly divide mixed-ancestry characters into the "You're really an X" and "You're really a Y" camps.
If I want to play a Khoravar, then telling me to just play a Human or an Elf and say they're something else misses the mark for me.
