Why do people say ‘don’t believe anything you read on the internet’?

I mean yeah it’s true you shouldn’t but some things might be true. You never knows. I read that an actor would be rude and obnoxious as a child star and I was shocked about how rude they would be. She looked like a quiet or sweet girl. She seemed very soft spoken and sweet in interviews. I read this a LONG time ago. My bro said, “You shouldn’t believe everything or every article you read is n the internet. Some people aren’t as nice as they seem or look or some person might of write it to make the person look bad. It might be two or it might not. Who knows. You can’t just a soon a person is nice or sweet based on how they look.”
I mean yeah it’s true. How can someone look nice end up being rude or obnoxious.

Why do people say ‘don’t believe anything you read on the internet’?
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