Because they don’t guarantee someone not getting infected.
Don’t go somewhere you don’t have to go (convention, large party etc.) if you don’t want to risk infection. If you have to go somewhere (grocery store) wear a mask to help reduce the chance of infecting someone else (as others should do for you). They say that two people wearing masks, even cloth, greatly reduces the chance of infection. I wear one for the sake of others and hope they do the same for me.
Or this could be the greatest government conspiracy of all time😎