Red herring,. Nobody says they are perfectly safe. The scienttific theory is the more people wear them, the fewer droplets will be in the air. It's a theory subject to testing and some tests show fewer droplets. Whether that's enough to stop an infection depends on the situation.
It has been suggested that viral load is important in determining the severity of a COVID-19 infection. Masks could possibly help reduce the load one is exposed to if the mask is effective in reducing the spread of droplets that contain the virus. Certainly masks are not a perfect defense, and all masks don’t provide an equal degree of protection.