I think it mostly comes down to the Author and his own experiences and wants, those will ultimately be reflected in his works, conscious or unconscious. Or he could have simply overlooked it. Or it is simply the genre trope that is prevalent in this kind of game, and it just is a bit of the formulaic approach that everyone follows... well because it works for the most part.I will be glad if you could share you personal guess.
And Oh God, MC and Penny actually kissed after the party. Like they remember they are normal human being.
Why are the girls behaving like they do? Because it is a fantasy, and not real life, simple as that.
But I don't think this has been written as a 'Friends with Benefit' Situation in mind as DanB has speculated, I think Cardi truly wants to go the exotic Harem/Romance Mixed Route, which of course only can work in fiction, hehe.
But to be honest, I don't remember the details of the relationships and how the unfolded between the MC and the Girls, has been quite a while since I played through the Game. In general, the game is pretty solid, I enjoyed the first chapters a lot - my absolute favourite was the 'Hogwarts' Hommage Chapter , kinda got a bit too futuristic / fictional later on, but still played it, for the ahem 'Plot' of course. But I find the huge Pringle Can Dong to be so distracting that I actually skip most sex scenes