When I was small child, our family lived in house without air-conditioning. We had screens on all the windows and an attic fan to cool the house. The gaps in the screens were small enough to prevent flies and mosquitos from entering the house. It would have been foolish to replace the screens with chain link fencing and expect that the bugs would still be kept out. A housefly has a wing span around 1/2 lnch, and the wires in chain link fencing are a couple of inches apart. The gap is about four times the size of a fly. A fly can easily pass through.
The gap between fibers in a typical cloth face mask is on the order of 2 to 3 micrometers. The coronavirus is on the oder of 0.3 micrometers.
The mathematics of the filtering is left to the Gentle Reader as an exercise.