While I agree monster x monster breeding would be nice, I can understand why it's not a thing for 3 big reasons.
1. Balance, you'd be able to snowball even faster if you could directly breed with monster girls rather than needing them to breed a human who potentially has worse stats.
2. For similar reasons as above, this would make humans and every other humanoid race completely worthless for breeding, since it'd be so much faster to just use monster girls than try to breed a good human from the monster girls.
3. Probably the biggest reason, it'd be a lot of work to do. It might be reasonable from a work perspective if it's just same race breeding, so Goblins x Shortstacks and Orcs x Onis, but if we're talking monsters of any race with monster girls of any race that's a lot of additional animations every time a new monster race is released. For monster x monster breeding they'd need to make 2 more animations for each combination, the actual sex animation and the birthing animation. So just to implement it right now for only same race breeding that's 4 additional animations, if we're talking no race restrictions that's 8 animations. Then when a third race is added that will be another 10 animations to make (new race with Shortstack, with Oni, with themselves, Goblins with new monster girl, and Orc with new girl), and that number only grows with each new race. You also have to consider this is on top of the 9 animations already needed for a new monster race, these being monster x basic Human/Elves (assuming basic Elves never get their own animations), Sisters, and Spellblades, and monster girl x MC and the milking room animation. So basically making such a change would pretty much double the number of animations each new monster race would need (not even including combat animations here btw), which would massively slow down development.
So while it would be nice to have, I don't see it happening any time in the near future, maybe near the end of development when they have more time, but definitely not while they're still adding more races.