It's a proven and commonly discussed psychological concept that the more people who are around to witness an incident, the less will take action. And by not taking action, it further supports the group idea that the problem isn't that bad, or isn't worth addressing, or they'll assume that somebody else has already taken action.

And I post this because it is also A PROVEN FACT (bolded to garner attention) that when people hear about this fact, they are more likely to give assistance in the future.