Because they do.

If you do nothing and say nothing when someone else is hurting someone,
then your silence says,
“This is how it should be.”
Even if all you say is,
“That’s not right,”
it’s better than allowing someone to continue hurting someone else, unchallenged.
Don’t stand by and allow
anyone to be hurt as if
they just don’t matter at all.

~Doe Zantamata