The issue is the lack of testing. Add in people who are contagious without showing any symptoms and you have a disease the spreads easily. If everybody wears a mask, that will include all those folks who don't know they have the disease and will reduce their ability to spread this disease.
The problem, of course, is the lack of masks. We don't have enough for everybody. It's a national disgrace that we haven't supplied our health care workers with the gear they need. Adding in the rest of the population makes it that more difficult. We need a nation wide, federally coordinated campaign to make enough masks and other protective gear (plus ventilators) for the whole country. There are plenty of manufacturing plants that can be converted (and many already have), but this needs to be coordinated nationally rather than in a piecemeal fashion.
