I am going to treat this as an honest question. If you want the “full science” answer, go here.
The quick and dirty: we get COVID19 when we inhale (or intake) droplets carrying the virus. The probability of getting infected goes up with the amount of virus we intake…yes, that means we can be exposed and not get infected. But the chances of getting infected go up with “dose” (amount of virus) we inhale …”more virus” means “higher chance of getting infected.”
Masks absorb the droplets that carry the virus. It doesn’t absorb ALL of them it it absorbs a high percentage of them.
Now think about how COVID19 gets spread: it starts with the infected person spreading droplets (sneeze, cough, breathing).
If the infected person is wearing a mask, the absorption is at the source. The mask is in a position to catch many of the droplets before they get out.
Now the non-infected person wearing the mask will have some of the “droplets in the air” filtered out by the mask..but the mask isn’t in position to catch as many as it can when the infected person is wearing it.
Still, the wearer gets their risk reduced.
When both the infected and non-infected person are wearing the mask, there is a series protection effect; the droplets have to pass through two barriers, not one. The droplets carrying the virus have to run the gauntlet twice, not just once.
For each virus carrying droplet, that is two barriers to elude, not just one. And the probabilities of getting through multiply.
Example: if a droplet has a 20 percent chance of getting through a mask, it has a 
Life of an old goat 