Why do people on social networks act as if they were "judges"?

I see it happen a lot, packs on social networks are created against this person or that personality.

For example there is a comedian who was accused of rape, people on social networks judge him guilty. But the final verdict was that he was innocent. Worse still, despite his innocence, people on social networks continue to judge him guilty.
Why do people on social networks act as if they were "judges"?
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