I'm not sure exactly why but I'm guessing it's because the beast gene is dominant. Meaning in any coupling involving beasts, their children would also be beasts.
But on the other hand, there's seems to be not a lot of beast siblings, most would be two like our twins here. So I think it's harder to get pregnant in the first place, especially if the bearer partner is a human.
Dude, have you read this series? The purpose of pairing a beast man and an omega IS TO GIVE BIRTH TO BEAST PEOPLE. There won't be beast people if it's not that pairing. That's why the clan has several omega spares and why the uncle is so against Juda and his cousin forming a bond. The chances of giving birth to beast people despite adhering
to the formula "beast people + omega" is still very rare.
Erm why are the Babies beasts and not humans? Is it because the author needs to give them more rights in order for them to take over their dad's whateverthefuck he does. Errrmm its giving Dear Benjamin where the ugly-ass excuse of a baby is blonde with blue eyes. Or maybe the 90 pills I took are getting to me.