Because they can’t.
Girls in this kind of story don’t want to get pushy and ends up being seen as a high-maintenance hassle by the guys they like.
Meanwhile the guys, especially with confidence problem, don’t want to risk getting teased because they can’t handle it.
It’s not just matter of feeling insecure though.
Rumi is the more social among the three of them. She cares more about having friends, and that came with being careful about everyone’s feeling and relationship. Her choosing to date Jun is stepping on her sister, and she’s feeling guilty about it, so now she wants to do something to pay for it.
Naori never have many friends, or watching out her mouth, and arrogant too, so she limits herself to people who can just laugh it off when she insult them (people like Professor and Chief). And she’s an all man for themselves person. If Rumi gives the way then she’s asking for it and got no right to complain if she became dead corpse of the loser under Jun and Naori who watches over the sunset in their happy ending. In other words, she doesn’t care, she wouldn’t give a hand to Rumi without being asked, and will just as likely to say “your choice, your problem”.
And like how Naori describe her family in chapter 1. Rumi and their mom are commies, they prefer collective activities and everyone doing it together. Naori and their dad are capitalists, they just do what they want without needing consensus.