Because jumping straight to sex, or other main sex scenes skips the point of build up. although I agree with some of your reasons, there has to be some build up, or the game will be too fast, and boring.
Look at all the game that make money. Most, if not all, have build up... slow burn.
Games with instant sex? Make shit for money. Facts are fact. And again, I agree with you on how some of those can be boring, most people don't agree with us on those opinions.
Gotta remember, most of us devs make games as a full time job. We have to have games that last longer than a few updates. Not hit it, and quit it games. Tehy don't make money. (Mostly) They don't support devs in the long term. Althought I am fully against milking subs too. Very against that bullshit.
Just my opinion, of course.