Because they're the ones who keep f**king up the world for their own personal benefit. Seriously, do you think we would have as many immigrants if we weren't constantly toppling governments in South America, bombing the Middle East and plundering Africa for all its resources? Never mind that America was always a melting pot from the moment English, Spanish, and French colonizers stole it from the Natives.
I'm sorry being the beneficiary of genocide and colonialism means you have to live alongside brown people, but some of us have actual problems in our lives.