I can see your point and understand what you're saying but just from my point of view, nobody cares about the main story in all these porn games. Honestly I love a good plot, but I'll go play a game or watch a show if that's what I want.
What I think is important and the best part of a porn game is the character interactions and dialogue during and after the sex scenes. Honestly most of the games I'll just hold ctrl and skip over the main story, after so long I'll just read a bit and I'll still know what's going on because it's just taking so long to get to the point.
And why are so many games main plot "your father was in the mafia and they're after us" then your character joins or does work for them or something stupid. It's the last thing I care about in these games.
Well, that depends on the main story, e.g. I totally care about a
pervert corrupting some poor girls... erm I mean, a scientist thoroughly educating mischievous students, as it's perfectly presented in "The headmaster", but when the main story isn't connected to the lewdness of the game, I would personally also not be that much interested in it, as there are definitely better ones in the non-lewd-genre.
What I like in AVNs is, that they show not only pure sex but more how it comes to the sexual interactions, especially in corruption stories and that's where you need a certain build up to explain, who the characters are and how it comes to their development.
I agree, that in many stories, this is much too much and most of the story could probably be deleted. But exactly to determine what can be deleted from a story to still keep the players/watchers/readers interested in the characters and their development is an art, that takes much experience/time/talent/skills... and is not that easy to achieve and when you are in the process of developing or writing, everything seems to be somehow interesting, as you are much deeper in the character, as a player will probably ever be. Writing too much is actually not only a thing, that you see in games. I read also lots of lewd literature on literotica and also there many authors which have interesting ideas, but the tendency, to barely bore you to death with extensive descriptions of things that just don't interest you when you just want to fap.
Alike the authors on literotica, many devs aren't skilled professionals, but rather interested amateurs and do all the writing, image-creation and programming alone. I think, this is the main reason why dialogues can be too extensive.
Devs who want to raise playtime with little time expense of their own, have better ways to achieve this, they can just add some stats, you have to raise by doing events again and again. A device, that is actually extensively used in some lewd games.
Regarding similar plot-lines: Some people just try to make money and hope, that what has worked once will work another time. This is something, that as far, as I know actually works up to a point. And one of the most famous lewd games is e.g. summertime-saga where this is part of the plot.