We need video evidence to show context and to be sure the evidence is in chronological order. With screenshots, you could take a picture of them before warning, then take a picture of you warning them. Then you could report and get them banned if you showed the warning screenshot first even if they listened to your warning and came down.
For context, sometimes you build up so you can build a sky bridge or to attack another player. With screenshots, you can’t capture their intent and it would like look they were just sitting up there when actually they were trying to fight someone.
So camping needs to remain with video evidence so we can be sure that the punishment is correct and not faked.