That'd be because in many cases it'd simply derail the plot unless the author already planned for that pregnancy. Also it'd often mean adding boring ass soap opera style drama which i'm not looking for in hentai games, are you?
Also going for realism is a bit of a catch-22 here. Because both answers (i.e. adding impregnation related content in some capacity, and not adding any at all) are realistically speaking correct.
Let's say you have a game that happens over 4 weeks, a common setup, and you creampie a chick in the first week, this means that even if she got pregnant, which is not in any way guaranteed (there's basically just
You must be registered to see the links
, pregnancy is statistically unlikely but possible here), since she won't be visibly showing her pregnancy until 3-4 months later the only way for you to know she got pregnant, is if she took a pregnancy test after the egg was fertilized (which might have been a few days after you creampied her, so let's say maybe she checks with a pregnancy test the day after but the egg isn't fertilized till the day after that, she wouldn't know she's pregnant, probably happens a lot)...
And then also decided to reveal to the player that she's pregnant which.... She might just decide not to do.
If we're making this 4 week game with maximum realism in mind, the odds of anyone getting pregnant and realizing it in the timeframe of the game are slim, them deciding to share it with anyone within that timeframe is even slimmer, imagine if you had a kid suddenly growing in you one morning, shit's a lot to take in especially if it was unplanned. She'd need some time to decide what to do about it.
And this all also assumes that it's an urban/modern time game, if it's a fantasy game, there aren't pregnancy tests.
And there are other things that can go wrong, the guy can be infertile, the woman can be infertile, she could be on the pill, etc...
So really... there's an argument to be made that often adding impregnation content would be less realism rather than more of it, you're not undercutting the creampies.
And even if the game goes on for longer than 4 weeks, even if it's years and you're making creamy little cumpies day in and day out, there are plenty of ways to rationalize how nobody gets pregnant if you wanna go down that route too.
Basically what it boils down to, devs do what they want, and in this exact case, they can do whatever they want without anyone being able to realistically call them out on it because both answers are correct.
Most games i've played devs don't add any birth control lines, and as I just established it's basically pointless, you don't need birth control to excuse shit. So the dev adding a birth control line would truly only be establishing how strongly the characters do not wanna have babies.