Just because it isn't 100% effective doesn't mean it's not doing anything. Exposure isn't binary--the quantity matters.
Healthcare workers have very little room for error because they're constantly exposed to large quantities of the virus. For them, blocking 80% of droplets from a cough is useless, of course, because they're surrounded by infected coughing people all day long. Even 99% isn't good enough for them. But for someone going to the supermarket once a week, blocking 80% might mean the difference between a deadly level of exposure and a level that their immune system can fight off.
Also, this whole "only sick people should wear masks" refrain rather exasperatingly ignores that we don't know who's sick and who isn't. That's exactly why this virus is so difficult to contain. Supply issues aside, the only way to make sure all the sick people are wearing masks is to get everyone wearing masks.
And your argument exasperatingly continues to assume infinite availability of masks.
The masks most help when people are coughing or sneezing significantly. That's really obvious last I checked.
Someone not coughing or sneezing is spreading the virus roughly the same amount whether they wear a mask or not. Covid-19 doesn't actually seem to produce nasal secretions like the flu does so surface contact spread isn't quite the same.
If you want to spread good advice, tell healthy people to start wearing their glasses instead of contact lenses. It's probably more effective than a mask on a healthy person--especially since it prevents people from poking at their eyes with dirty fingers.
Except the part where I wrote "supply issues aside". Of course there's a limited supply. That doesn't mean masks wouldn't be effective if we had enough of them. I think we should prioritize making more, not just for healthcare workers, but for everyone.
"Someone not coughing or sneezing is spreading the virus roughly the same amount whether they wear a mask or not."
Scientific consensus so far is that coughing, sneezing, and talking is responsible for the majority of spread. Surface spread is secondary.
"probably more effective than a mask on a healthy person"
Again, we don't know who's healthy and who isn't.
Have you seen the pictures of healthcare workers using "effective" mask protocol? Have you also noticed that said protocol almost always includes goggles?
If I can't trace the seal lines on your face for an hour after you remove your mask, it isn't doing shit for you as a healthy person.
If you are sick, however, that's a different story. The mask prevents droplets from being spewed out of your mouth at 120km/hr (70mph for the heathens) when you sneeze.