The standard argument is that a mask may give you a false sense of safety which results in subtle behavioral changes that are a net-negative. Additionally if the mask inhibits breathing you might take deeper breaths which is also a net-negative.
That standard argument is a myth simply used to shape opinion in the context of a shortage of masks, to redirect the existing one to health care workers.
In countries where the epidemic is controlled, pretty much everybody wear masks, including handcrafted one in case of shortage. The goal is not to prevent from being infected, but to prevent infecting other, by limiting the amount of saliva you spread everywhere. Especially since tons of carriers are asymptomatic, or starting to be contagious before becoming symptomatic. And even a poor mask in front of your mouth is better than nothing, at least if everybody wears one, and if you don't become careless on other measures (distancing, washing the hands, etc.)
So there is an availability and potentially a cultural problem, but that can and should be fixed. Because this will be far less costly than infinite lockdowns.
