Wouldn't there be more hybrid races lorewise?

[Susanoo]

Yes there should. And the already present hybrids show that there is no restriction to the mix.

If I had to go for an explanation, I'd say that those mixed couples don't actually give birth to hybrids but to children of one or the other race.

Of course my only argument is that they never actually made a relevant model for the hybrids present ingame. Arator looks like an elf and Garona and Lantresor like orcs.

Kalecgos's humanoid form is probably the only somewhat "unique" model of a half-elf and he still looks mostly human.