Let me repost the edit:
Mask does't stop all covid-particles from getting out, it just limits their flow coming out of you.
So if 1000 particles of Covid-19 are traveling out of your throat, part of them would settle on your mask along with the moisture that carries them and the remainder would get through on finer particles, but because your airflow is limited, they would travel much shorter and settle on the ground faster instead of gliding around in the air. Without the mask more particles shoot out and their spread has much more reach.