You don’t see 100% infection rates when a positive person is in a a room. Reducing the amount of particles inhaled in such circumstances must be of use.
You would on the other hand expect a 100% fatality rate if you fired 123,000 bullets at each person in a room. So your analogy is faulty.
I hear this all the time "masks have proven to be incredibly effective"
Have they? Data didn't really bear that. And there are tons of studies that day, for the lay person, we don't know. Or they say n95 and surgical masks may be effective but cloth masks are an unknown.
The infectious dose of SARS-CoV-2 is as low as 10 viral particles and a single cough can release 123,000 viral particles.
If I shot 123,000 bullets at you, would you be happy with a shield that protected you from 80% of them?
At any rate, masks have proven to be incredibly effective, but the truth is that we don't know exactly how... common sense has not been particularly informative in understanding the spread of this virus.