Help! someone help!"... What? No one? But there's a bunch of you? Do something... Sigh... That, in a nutshell, is what psychologists refer to as "The Bystander Effect".
It is something that happens when a large group is present, even if you can see something is going wrong, even if you can clearly see that someone needs your help, you will hesitate to do anything, and most of the time just stand there, hence Bystander Effect.