It is about viral load. Masks do not stop all the virus for the exact reason you stated but a good quality, properly worn mask can decrease the amount of virus you breathe in (at least temporarily). Testing has shown that effective time to be 15-30 minutes in fairly close contact (about 3 feet) up to an hour if the infected person is also wearing a mask. Everyone probably has breathed in some COVID virus at one point or another but if you do not breath in a sufficient amount, you will not get infected. That is why social distancing actually works. If you are far enough from an infected person, the viral load you would breath in is not sufficient to make most people sick.
At risk people should consider wearing a mask if they will be in close contact with others. You don’t need to wear a mask in most public places since air circulation is usually going to disperse COVID enough that you will not get enough to be infected (unless you have really bad luck). COVID is highly contagious but not nearly as bad as some stomach viruses which can make you I’ll with just a tiny exposure just from touching something and not cleaning you hands properly.
Masks are nonsense. Just stop.