Because even without symptoms, you can still be spewing virus. That's how it spread in my office. One individual with zero symptoms was spewing the virus big time. Passed it to 3 other people in very very short time. These people were not in close contact with each other (probably 5 to 6 feet apart), but were not wearing masks. The one who spread it, also developed the Covid pneumonia, chest scans showed signs of the "glass" in the lungs, and will now probably have permanent damage.
Ultimately, wearing the mask is more about preventing you from unknowingly spewing virus than you getting the virus from someone spewing it. And frankly, it's not a big deal to wear one when out in public.
People have made this issue out to be so much more than it needed to be. Am I in favor of government mandates, unconstitutional limitations on rights? No, but I am for common sense and courtesy.