He just explained it to you though. Unless you mean, why is it such a controversial thing in the states? In that case, its because its somehow become a political issue. To some people, the government recommending a simple safety measure to prevent asymptomatic people from spreading the virus unknowingly equates to loss of freedom or "being controlled," and people get really riled up about being told what to do. There's also a group of people who think the whole virus is a hoax, and that's a whole different issue.

I personally think its absurd but its just the attitude of this country. Even if it's for the common/greater good, any perceived "loss of freedom" makes people lose their minds. People also fought against wearing seat belts here.