They all care about him because he saved pretty much all of them, taught them how to survive, etc. They don't want him to go seclude himself away from everyone in a cave somewhere where he'll be lonely and forgotten, which is absolutely what he plans on doing.
The WN goes into it a bit more by showing the POV of the girls, they have regular "strategy sessions" where they try and figure out how to keep him from retreating into his shell and away from everyone else. For example, they try and figure out if he's gay (some of the girls are REALLY hopeful for some BL between him and Ota or the Jocks) by dropping hints and fanservice around him, but he's so dense that all it ends up doing is getting one of them the nickname "Nudist Girl" when she strips in front of him to use the bath and he just thinks she's a weirdo -- which gives her a complex about not being sexy enough to get his attention. Meanwhile he's thinking they've all gone feral because they keep "forgetting" he's a horny teenager.
And of course, the class president is outright in love with him.
