I'd always assumed that the lack of more distinct half-elf and half-orc cultures was due to their relative rarity. I think even in the Half-Elf description they're described as either mostly accepted in human society or slightly more apart in elven society. This also informed for me how other races might procreate, for many it was either impossible, exceedingly rare or only possible one way, like in Dragon Age or Elder Scrolls and maybe the Witcher too, I can't remember.
I wonder what ramifications on the FR setting this might lead to, I still tend to look at the world as being fairly parochial until you get into the extraplanar realms, where the term cosmopolitan takes on a whole new meaning.
