Because you're (not you, but people in general) are dumb and overly trusting.
This is the correct answer.
Because you agreed to do so under the terms of conditions of that website.
Look I understand you point from a legal standpoint, but do you really truly believe even a small fraction of FB and IG users actually “agreed to do so under the terms and conditions of that website”? They just clicked whatever was necessary to create their accounts. I doubt there was much affirmative agreement going on there.