Because it is settled science.
Whether or not masks work is a physics problem. The virus is transmitted P2P (piehole to piehole). Anything that interferes with that trip lowers the probability of infection, Period. It can filter out the virus (best) or reduce the flow that the virus is embedded in.
Get a friend to stand nearby, and cough or sneeze in your face. First without a mask, then with. It’s obvious.
Be in a room with a smoker with and without decent ventilation. You’ll see the difference.
Mask mandates? Compliance? That’s a social sciences problem.
I’ve seen the plot of Japan and Hawaii, how they controlled it for months, then it took off. It’s almost
like a new, much more contagious variant showed up.