Probably because almost 100% of the time the "half" part comes about from the mother being raped by the labelled bit.

You aren't going to find many human guys that will fuck an orc woman, and probably more importantly, orc women won't want human men who are physically weaker than not only orc men but also probably orc women!

Garona Halforcen, [subtle no?], from WoW, result of draenai female being raped by orc male.

The twins in Icewind Dale 2, result of human female being "impregnated" by a demon.

It's pretty much always rape and when you start noticing that it then brings up a likely inherent label of the races involved in siring the offspring.

Hence, racist. Because these idiots are too fucking retarded to realise what they are doing most of the time with their self insert OCs instead thinking "uwu, Imma half-dragon tiefling with horns and a tail, swish-swish!" that either required a demon to fuck a dragon, or a dragon to fuck a demon, and regardless probably needs the dragon in some kind of non dragon form if the resulting tieflings are somehow still humanoid in appearance.