They got magic and stuff so why won't they believe him being isekai'ed and possibility of other worlds existing
From previous chapter comments, written by me, re-shared by me:
The plain fact is that the more that is discovered about a particular thing, the more certain everybody becomes about what is and is not possible regarding it.
Sousuke maintains an open mind because he doesn't have a lot of magical knowledge, but he knows magic exists and can do some wild stuff, so when someone tells him something incredible, his reaction is, "Well, why not? That could be the case, so let's go with that until I see what's what."
The wizards have a
lot of magical knowledge, and accordingly have a lot of documented information about what magic can and can't do, so when Sousuke tells them something incredible, they throw him in jail. The only reason Kozumi believes him is because she saw him doing things that she "knew" were impossible, which showed her that maybe she doesn't know as much as she thought, so now when Sousuke tells her something incredible, her reaction is, "Well, maybe? I don't think so, but let's investigate a little more."