Why do you think that stereotypes exist?

So i was doom scrolling as usual on insta (biggest first mistake and came across an Asian lady cooking a traditional dish and i was like dam she making me hungry only to find out not everybody felt the same??? Lowky was dissing her and sayin mean shit chat.. I gen dont understand why stereotypes exist? and just wanted to know how are stereotypes (positive or negative) harmful? DO educate this poor soul cause honestly google aint helping rn. if u have an experience this would be even better (to help me understand) honestly..