Masks are often recommended as "source control" -- if an infected but asymptomatic person wears one they reduce the risk of spreading the illness to others. The bit that goes unsaid is "a good quality mask, such as a 12 to 16 layer DIY cotton mask, or a properly fitted N95, worn properly, with excellent hand hygiene when putting it on or taking it off".

The problem with that framing is that most people quickly drop the "asymptomatic" bit, and so now we have people with symptoms going shopping wearing a mask. That mask isn't going to do much to protect other people, especially when you consider it's likely to be 2 layers of cotton, poorly fitted, with poor hand hygiene.