Really? Your humans build underground cities and are united in their hatred of a specific enemy?
Look, I'm not talking about the quality here. Just the fact that halflings do stand out here as being 100% replaceable by humans without any changes other than about 3 feet vertically. Dwarves and gnomes might describe some humans, at best, but, the halfling description is pretty much indistinguishable from how you would describe most of humanity.
Let's flip this around. Dwarves live underground and Duregar (which are also dwarves) live even more underground. Elves love the forest but some elves also live more underground. Gnomes live all over the place, and some gnomes live more underground.
The elves and gnomes that live more underground changed to adapt to their environment and presumably build cities where they live.
So what keeps humans from having had an offshoot go more underground (just like dwarves, elves, and gnomes) that carved itself cities there BESIDES an arbitrary decision by the writer of the fiction?
Why do all tieflings look like stereotypical devilish humans? Why aren't there dwarf tireflings and halfling tieflings? Are there normal human looking tieflings that have no devil features in a book? Why not?
Same question but for genasi. Why aren't there dragonborn or tabaxi based genasi?
Same question but for half-orcs. Where are my tortleorks or Orkliaths?
The answer to all these questions is that if you want those things in your game you can add them, but the PHB has a very limited space in which to introduce and stat out the races it has decided to include. Part of the design process did not, apparently, include a requirement that "This thing only functions as written and can't be replicated in any other way or else it's not good enough."
I can't think of any choices that can't be alternately explained by the story "Humans moved here or did such-and-such and we're changed by magic to be this new thing. You already accept that logic for tieflings, so why not couldn't it be applied to dragonborn or warforged?