Well... I'm not imposing that I speak for everyone but I think there is 3 reasons at least:
1. Fetish.. unacceptance - Western artists(i mean of course those artists who makes animated stuff) doesn't really like to make stuff about pregnancy not even with a hint for that... may sound strange but I remember only... crap I can't even count the exact small quantity of people who makes any good preg-relared stuff. But I can swear there is not much of them. Zetar02 with her PX:LPD, Lusty with his/her birth animations, NearFatal(GhostHouse and specially MUA)...
Why there are not listed breeding season? Too simplified breeding and the only preg content "happens" if your character carrying some monster bb in her(of course if you are playing as a female character.)
2. Mechanics - artists are usually suck at any "techy" stuff due to their "artistic" nature and not really welcoming towards cooperation since this may ruin their "idea". So they don't really want to mess with some unknown concepts and usually picks up something that is well known and simple(possibly). That's why there is a lot of PvZ like games(linear enemy path, 1 direction of attack) or Breeding Season like games(count by yourself... you can start with Breeders of Nehelym).
Making them put their fingers into something that is hard and unpopular is almost impossible.
3. Quality - Name me at least one western 3D porn-game which has at least the same model quality like in those numerous HQ videos. This is where unity(and not only Unity) eventually never wins because there is already divide in 3D community- there is artists who makes videos. And artists who makes interactive stuff.
You can't expect VGerotica to cooperate with some guys and create an Porn Version of Nier, right? She simply turn you around saying tha gameplay models are not her profile.
But you can hire an almost any 2D artist with a lucrative offer, especially if he already have an experience in animated images(*carefully points tail at Kabier BareBackStreets), but sometimes even inexperienced can do something cool too (*this time tail is pointing at Crescentia and whole Wolfenstahl project with their Paperheads.)
Aaaand no I'm not talking about japs because only god knows what kind of mess is happening in their porn-industry.