Philly does have a litter problem, and "don't litter" isn't the answer. Even if no residents ever littered (they do, of course), there are other issues. Visitors litter. If it's windy on trash collection day, stuff from recycling bins and blows around. Stuff from left open commercial dumpsters gets blown around, too. The trash collection people are pretty good in my experience, but they're not perfect about picking up stray items. Who knows who dumps in vacant lots, but they do and there isn't much neighbors can do about it except call the city and hope for the best. That stuff blows around, too.
In a densely populated area, stray trash is a public problem requiring a collective action solution, no matter how diligent the residents are.