Almost all contagious people never stay at home. Based on my experience, people with colds come to work and go about their business.
That is anecdata. Maybe valid for, say, January. Starting Febuary, that changed. Again anecdata, but In Germany you got up to 2 weeks of sick eave for potential COVID-19 symptoms without seeing a doctor.
I must be unlucky because my colleagues almost always come to work sneezing for a few days until (if!) they take sick days. This was right before the pandemic, when the situation was getting hot in China but not yet in Europe.
The point is that when almost all contagious people stay at home, we don't need community masks. We only need them because contagious people are asymptomatic (that's new!), tehrefore don't know about their status and therefore go out in public.