A Question I've Always Had On The Races...

[Iscah]

Excuse me, while I understand the point you're trying to make, I disagree that a Hyur and a Miqo'te are like a Human and a Cat.

They didn't say that, though.

Even in the real world, closely-related species (like the various big cats) can interbreed but the offspring are often infertile.

And as Tsumdere pointed out, there are real physical differences between the races that may not neatly combine if their bodies have different shapes, particularly ear placement (and necessary difference in the underlying skull shape) and tails.