Because we walk around our own cities for YEARS and see less of this then we do on a random walk on a single day.
So have I - and downtown SF is worse (in terms of littering, human excrement, aggressive panhandling, street crime) than the problematic parts of NYC, DC, Philly and Baltimore, worse than densely populated capitals in Latin America or East Asia or Eastern Europe. It's better than Indian cities but that is a very low bar for one of the world's richest cities.