The story is as old as Kitty Genovese. 1964.
It’s a social psychological phenomenon known as the “bystander effect” or “diffusion of responsibility”. It’s a result of liberalism. For example, liberals find it (much) easier to vote to have their neighbors’ money confiscated by government and handed over to “the needy” than it is to actually take responsibility and do something themselves. Somebody is stabbed and dying? The government will take care of that problem.