For example, Marx said that more capitalism logically leads to subsistence living conditions for the workers. He gave arguments for this. I think they are mistaken, but if you are wondering why people think profit is evil, that's a reason. Marx said that without fraud or coercion, and just free trade, most people will have very bad lives.
And, yes, obviously one can live in the US for a lot less than minimum wage, and be doing a lot better than the subsistence conditions Marx had in mind from his own time period. So Marxism is blatantly, factually inapplicable to the US (whether that's b/c Marxism is false, or b/c the US has been saved by it's non-pure capitalism with govt intervention is not obvious just from the plain facts). Why people are blind to this is a tough question.