Because most people that change their behavior that drastically, going from a caring guy to not caring at all, has had some big events changing them. We have not seen anythign that will makes us believe that the MC has changed that much, to just not care at all about the girls he is with when he cared that much when he was younger. Without a few events changing him and his way of think, it is hard for a person to change that much.
Also it is a game and the MC is a character, he need to be consistent. I can make a character be a funny guy, it will need a good explanation if I show the same character years later and now he is a dick to everyone. Same thing happens with the MC, he is a caring guy, everything that has been shown to us makes us think he cares, and now that years pass he suddnetly doesn't care at all about the girls and is ok with sharing them, without showing anything that makes us think he has changed, like an event or something taht marked him? Yeah it doesn't match at all. the excuse of "it has been all in the past he has changed now" doesn't makes sense, because we haven't been shown that change, in fact if he didn't care about the girls in the present why would he be remembering anything of this? or even be bothered by it? He doesn't care right?
It's like making a character that has an harem and is fucking all the days, and when the time pass and you show him again, he is a celibate that don't come off his house, because reasons and time passed and people change with time, would that make sense in the pioint of a character? would it be normal to do that huge of a change in a character just because most people don't act in their 40 like they were in their 18? It wouldn't make sense, neither will be the MC going form caring guy who is/was affected by all that happenend that even in the present still remembers those events, to I don't care about any girl and I would happily share them with others and watch how they fuck, because years has passed is a nice and self explanatoire reason.