Why do people judge others when they don't even know them?

I'm an introvert and there are some people who think I don't talk to them because I don't want to talk to them but the real reason is I don't know how to start a conversation. Some of them don't even know me and start hating me for no reason. They want me to act like an extrovert, I mean how can I act like someone I am clearly not.
Why do people judge others when they don't even know them?
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