I dunno, but you're right. Out of all the weird fetishes, it's the one I see people mention the most.
One of the issues with pregnancy is it's one of those kinks that isn't just a sex scene you can forget about. You usually need to add gameplay/story elements to allow it. It's a kink that has gameplay/story consequences. It's not easy to just throw into any game like most shit. A footjob is just a cg/sex scene that can be forgotten after its over, pregnancy needs consideration basically. What you're saying can add to story telling also means distracting from other content and possibly restricting the dev's options for how to continue things.
Talking about realism:
1: It takes 9 months to have a baby, most games don't last that long (Rance games seem to disregard most the grindy aspects as canon and just have key events pass time for example. I think the games usually take place over less than a few months.).
2: For a good chunk of that time and time afterward, there's a period where the mom can't fight.
3: It can end adventures. Normally when you have a child (Especially for women.), that's the end of what makes most games a game.
4: What happens afterwards? You just ignore the child created just for a pregnancy kink? Needing to tie it into a story and dialogue constantly can alter the story, possibly in a way that the dev doesn't want it to go. Wouldn't it be weirder to just never mention it? Rance games are about as casual about it as can be about it, thanks in part to having the luxury of the main character traveling around and not having to worry too much about the destruction and babies he leaves in his wake. He's also an asshole that doesn't mind leaving children around, it's a lot more difficult to explain if the MC isn't a massive dick, lol.
Only way I can really see it working in most games is at the end of a game (Which might not be the way the developer wants to take the game), or you just banging some random pregnant chick, which is kinda weird if you don't have a pregnancy kink, lol.
TLDR: Pregnancy is hard to put in games. It's up there with NTR. What people seem to like so much about it also makes it far more difficult to add into games "properly" (that don't revolve pretty heavily around it anyways). Having babies, and going grand adventures, are kind of like oil and water.