Because he lets everyone walk all over him. I lost count of the name calling and insults in the bunker room from Ruth and Ghost. Was that just 'banter' at the time? No...it wasn't.
At that point he was essentially being judged by the team as a prospective recruit. In the sisterhoods shoes, I would most definitely toss some insults his way, just to gauge how he reacts. Especially knowing what his power is. If he's hot tempered he's a threat. I wouldn't trust someone with a short fuse to join my team and definitely wouldn't trust transferring my power to him.
And from the MCs point of view, he actively wants to join their team. Why even chance ruining a once in a lifetime opportunity to not only join with his favorite heroes, but to even potentially fuck em all? It was basically a job interview for his dream job. He wasn't going to risk blowing it by being hot headed.
Soon after his power became apparent and he was chatting with Moonsong about it others with a similar power to his came up in the conversation. I can't recall the specifics but do remember it wasn't good. Something about them being corrupted by the power they had, and needing to be taken care of. So the MC was, in essence, warned right then to watch his temper, or else the consequences could be severe.
In school he's at the bottom of the totem pole, being powerless in a school full of powered people. So yeah, I'm sure he had to endure more than his share of bullshit and learned to bite his tongue so things don't get even worse. He has powers now, but has to keep them hidden. And even if he didn't, the principal clearly doesn't like him and has the power to send students to the farm at his discretion. Which he uses frequently. So him rocking the boat at school is a terrible idea.
Essentially the MC is a guy who's been conditioned over a long period to keep his cool because he knows if he doesn't, things could get very bad for him, real fast.
We've already seen this changing somewhat with how he treats Lucia, especially if you're on her manipulation path. And as his confidence grows along with his powerset I don't see him remaining so passive. But I also don't see him becoming the type that'd get angry over some insults.
With his friends there's no hostility, it's all friendly jabs at worst.