Why people consider me as stupid?

I feel like people always assumed me more stupid and would do some shit toward me, and when I would confront them about it, they say most ridiculous things. Like, they will say something so stupid like I would eat it up cause I’m stupid. Why it happens that way?
One time I was mad that one girl who copied my work without permission, and other girl who wanted to do same, said I’m overreacting. Why people assume I’m so stupid that they assume that they can change my mind or teach how I should react?
Or my boss who didn’t pay for 2 month of work, also wanted to manipulate me to think that I’m the one who is crazy and overreacting. He’s arguments was so stupid, he considered that I will just accept it cause I’m not capable of thinking?
Why people consider me as stupid?
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