How often do the different Races inter-mingle with each other?

[PArcher]

when logically (given all the mixed-race romances we see among NPCs) they should be all over the place.

Just because there are mixed-race couples doesn't mean they are able to produce viable offspring. Look at the real world and inter-species breeding; most half-breeds die before birth, and many are born with serious defects that kill them shortly. There's only a few that are able to produce viable offspring, but even then they're still often "broken" (mules, for example, are sterile and can't breed at all)