Masks become biohazards in very short order. Especially if the wearer is exposed to aerosolized pathogens.
As pathogens are presumably captured by the fabric, they are concentrated and kept in cozy moist conditions.
As the mask is touched when it is adjusted; when the mask is shared with family members; when the masked is used several times over days; the concentration of pathogens makes for an exceptionally heavy viral load thereby increasing the risk of cross contamination via surfaces.
There is ample research going back to the late 70s showing that masks are not effective barriers against infections even in surgical theatres.