Alright, then he should've put the effort into getting to know them. I'd understand if he didn't really like them that much, but he clearly did, as evidenced by him being able to fall in love with almost all of them. He could've definitely put the effort into forming a bond and it wouldn't have even been that far out of his way, yet he decided "fuck it, my only friends will be whoever my dick's in and their friend". The introvert thing wouldn't really apply here, as these are people he already knows on some level and not complete strangers. The worst part is that these people would benefit greatly from his friendship, and his decision to just not care is hurting them.
You're arguing two different points at once here.
You're saying he needs to put in effort to make friends with everyone then you're saying he's making a conscious effort NOT to make more friends. Which is it?
I'm going to ignore the idea that Hisao is actively choosing not to become friends with people, because honestly, that idea is straight out completely and totally stupid. Hisao doesn't actively chose not to care, in fact there are more than a few scenes that show he's somewhat worried about people that aren't his friends, but he doesn't know them and doesn't feel it's his place to meddle in their business. Hisao actually thinks this when dealing with Miki, for example.
Hisao doesn't actively dismiss people, so let's go with the argument that Hisao
should be trying to make friends with them. Of course if you want to go that route, the question is, why should he?
Maybe it's just me, but I'd assume people become friends not out of a sense of obligation, but because they want to be friends. Hisao doesn't WANT to be friends with the others. It's not that he doesn't care for them, he's just happy with how things are for the most part. He's never had a huge social circle and he's content not to have one. So, why should he make friends when he already has enough? To help them? Go into Hanako's route feeling that you need to help somebody and see how that works out. Go see how Emi reacts to being told that she needs somebody.
Hisao is fine with only a couple of friends, so WHY is that a bad thing?