That’s a common misconception of how masks work during an epidemy. The main reason to wear a mask is not to be safe from other people. It’s to not spread the virus (that may not cause any symptoms yet but be present in you) to others. That’s why doctors wear masks during surgeries - to not harm the patient. A proper mask works better and can protect you as well, but a cloth mask can limit the amount of breath you spread all around you and can be effective enough to limit the spread of the disease. So it’s not the same situation as with grain dust, where you need to protect yourself, not the others.