The story doesn’t elaborate on why a 13-year-old girl was out on the street in the first place? Big hole in the story.
Not an excuse, but I’d venture that some people passed by because they didn’t want to be accused of trying to molest a child.
The potential for liability is endless. It could be also a prank, or a setup for extortion, with her "big brother" hiding nearby and ready to accuse you.
Today the most reasonable response would be to call 911, anonymously, and then remain in the area to make sure they find the victim.