A lot of them are probably event swag, as previously mentioned.

The other thing is, a lot of colleges don't have any kind of storage for people to use during the summer (and a lot of people are graduating seniors anyway), so everyone has to pack up everything and take it to their parents' or to off campus housing or to their new city. If they've got to move everything they own, maybe it doesn't make sense for them to move grocery bags. I don't think it's green washing so much as basic practicality.

On the other hand, yeah, some people do treat reusable bags as sort of...trash adjacent? Especially the kind of low quality swag ones or the ones that are sold in the checkout aisle. If you use it ten times it saves you money, which is great, but at the end of the day there's no attachment to it.

I think it would be great to try and organize a move-out sale or donation event. When I was an undergrad each dorm had their own unofficial free stuff place--in mine it was a table in the lobby--where people left usable things. But I now work on a college campus, and the things people throw out (especially the rich international students--stuff like working TVs and computers!) is mind-boggling. We've at least managed to get donation boxes around the end of the quarter, which I guess is a start.