I don't think it was always this way. I remember visiting Baltimore as a kid in the early 2000s and being surprised by how clean Downtown was. Maybe 6 or so years ago, there was an article written for the in-flight magazine for Southwest Airlines about Baltimore and the author raved about how clean the city was. People obviously don't even consider negative consequences for littering to even be a possibility. Baltimore just generally does a terrible job of incentivizing pro-social behavior.