Because it literally does not stop the spread of the disease. It stops people from spitting on each other, which would be the same effect distancing has. We all learned how to cover our mouths when we cough and sneeze at very young ages, as well as basic hygiene unless you were raised by animals. If you would like a system that would in fact protect you from a communicable disease, please see the biopak 60p here: https://www.warcollectiblesforsale1771.com/searches/oxygen-rebreather-.html