Facemasks are _also_ useless at preventing viruses from propagating outward. The cheesecloth-condom simile applies both ways.
That would be true if the virus particles were somehow floating around on their own. They're not. They are largely contained within liquid droplets associated with your saliva, mucous, etc. These liquid droplets are expelled out your nose when you sneeze and will float around in the general vicinity for a few moments, which is why they say you would only get the flu if you were within 2-3 rows of the infected person. The facemasks - worn by the infected person - do a great job of stopping the liquid droplets from a sneeze. I suppose so does covering your nose with your hands - and then immediately washing your hands - but the facemask might be more practical if you can't get to a sink without first spreading the virus somewhere else.