Which is a little weird, because as far as I'm aware Patreon comes down just as hard on "step"cest (cause they're not idiots and realize it's code for full incest).
You'd think devs who want to make incest games would just arrange for an "unofficial" patch with every update.
Pregnancy is a different ball of wax. It's hard to implement a nine month process in a game where the changelog for Update 15 says something like "added first half of Day 8". It potentially adds an explosive amount of rendering work if the player can independently choose whether or not to impregnate multiple love interests (have three girls in a scene, and need eight variants depending on which ones are pregnant). It can (really, should) add major story complications if the game continues through the pregnancy, and if the story ends with the pregnancy then logically the pregnancy can only really happen near the end.
And at the end of the day not everyone's actually into pregnancy, even if they do like incest.