That’s also their logic for it being racist. Leftists insist that there is only one race of humans because we can all interbreed. Therefore, it would be racist to call elves and orcs different races if they can make hybrids.
(Of course, in reality, this is the definition of species, not races.)
Also, half-dwarves are seemingly never a thing (except in the Dark Sun version), so half-races make even less sense because of that.
Rats and mice are very similar, but they can not create children together.
(Of course, in reality, this is the definition of species, not races.)