You've not been looking down then. I used to play "Human or Canine" while walking from the train to my office in Miami. I like to think I got pretty good at telling the difference after a while.
I don't think I'd consider any other city in the Bay Area a major US city. Maybe it's just California then? I've literally never seen this on the east coast, mid-west or south.
San Francisco is worse than any city I've been to but it's hardly the only one (major or not) with shit on the sidewalks. On the East Coast I've run into shit in Brooklyn, for instance.