Two things make this disgusting: First, it’s always the same people, and second, all the bystanders do nothing. Is the second thing because they belong to the first group?
“all the bystanders do nothing” ... well, somebody has to get the video.
One reason nobody helped the victim is that they would be charged with a crime against the attackers, almost certainly.
Another reason is that the attackers, and perhaps some of the watchers, would then attack the defender.
If the bystanders are White they know if they do something the full force of the Law will fall upon them. And the Law will do nothing to the attackers.