Because it is weird.
It is weird, and niche. It is -Not- the same as two humans of different races having a romance, because they are the same -Species-.
Inter Species romance should be very, very, very unlikely. (This is part of why the Windrunner Sisters are just ridiculously stupid characters)
We don’t have a real world frame of reference, because there is only one sentient and sapient species on the planet, and that is us, in all our wonderful variety of ethnicities and colours.
Azeroth however is very different. There, there are about 80 or 90 Sentient and Sapient races? It’s hard to keep count, I’m obviously including NPC races here, in fact yeah, there are loads of them, absolutely loads.
Our laws just wouldn’t apply, our sensibilities just wouldn’t apply, however, we do get some examples as to how these things are viewed in game lore.
Human and Gnome seems fine. Seems weird. And is probably going to end in some heartbreak for the Gnome in the relationship as their lifespan on average is three times as long as the human partner.
Orc and Draenei is best just drawn a curtain over and going “Nope, not touching that subject” But equally, the offspring -generally- are not regarded that well at all.
Oddly, Humans and Elves is the most controversial. All humans find Elves desirable (Apparently, according to Blizz) whereas by no means do all Elves find humans desirable (Hence the Windrunners being freaks) The Offspring of such a pairing though, the Half Elves, properly falls into ‘Uncanny Valley’ and both Humans and Elves regard them with scorn and residual disgust. The stories about Alodi receiving the hatred from his human contemporaries, and Kalecgos in his Half Elven form getting violently disgusted reactions from Elves of Quel’thalas kind of show how it should be. The Windrunner Offspring are probably lucky because of who momsy and popsy were and because of Hack Writing.
The reason it is rare on Azeroth, is probably that. Social opprobrium. Difference is, it would have reason in that world, whereas attitudes towards mixed -race- couples in our world have no reason or basis.
“It’s hard to form a deeply fulfilling romantic relationship with something with the relative age expectance of a pet Hamster”
Should have said every Windrunner ever…