I'd agree that devs and players could just assume the girls are on the pill, but I have seen games where devs have this type of dialogue after nearly every sex scene, sometimes even with the same girl more than once,
You know, you reminded me of something. My game, even though is more gayer than a F95Zone convention, still has the vaginal sex tag. Why? because it have one scene with a trans man. Unlike 99.99% of the scenes with biological men in my game, that scene with the transman is a little bit tricky to get into the sex part (partly due to my audience not liking pussies) and in one of lines AFTER the act, the transman jokes and scares the MC reminding him he could get pregnant, giving him a death scare, but then he assures him he usually takes the pill beforehand. I'm planning to have at least one or two additional scenes with hot transmen I've seen in porn, it's kinda like an obscure kink I have (Keep in mind the only vaginas I've ever seen are those in porn films and internet).
probably the same reason as to why they don't implement STD's in their games

too much work
That was part of the original design of my game. I planned them as debuffs, that get worse in time, leading to a game over if they weren't treated at the hospital. I even had the condoms as items in game already.
But I decided against it. They don't add anything, to gameplay, story or eroticism, and it's a subject that triggers me a little. so I didn't implement that part in my game. I'm planning to have an entire storyline based on male preg on my next game (what?? the game is magic based, a wizard did it) but I'm not coming back to the STD feature.