If you have seen everyone wearing a mask, everywhere, at all times, if they are less than two metres or six and a half feet, apart you would be justified in asking "Why don't masks work?". (and that is the very minimum, A fit of coughing, or a conversation shouting at someone, or sitting in a confined space where your breath could be leaking out of the sides of the mask and hanging around, could carry the virus further than two metres)
If you have seen people without a mask, then it is obvious that just the authorities saying that people should wear a mask isn't going to be enough.