Because he's not good enough for the mainlines. The story said this multiple times. He would be better helping people with easy quests and the party wants to go for the harder ones.
Honestly this trope is stupid mainly because people make it a show out of it. (that and some other stuff, like rape allegation or stealing to make it "more tragic", Shield Hero style.) It's just conflict of interests of the end of the day. The Thief girl is right, tho. They can still be friends at the end of the day.
That problem isn't that they kicked him out. It's always how they kick them out. In this story. Yeah, he was weak, but one of those said it was because bad team cohesion and because raid wouldn't help him train. Both of those are things team members should have pushed for. And then on top of it all team members that supposedly care about him, and have fought together with their lives on the line, abandoned him. These teams are supposed to be their livelihoods, and they just left him without any kind of transition period.
The mangaka wrote an intro he seems to be trying to soften up, but it comes off as awful writing. And you seem to just accept it.
Either this writing is trash, or those characters are. And if it's the characters mc shouldn't want them as friends.