Maybe because the interiors of their cars have been exposed, such as by carrying groceries or having unprotected hands on the steering wheel. Maybe just to be sure they don’t forget it. Maybe to go from home to destination and back without losing whatever protection they had at home.
Maybe just superstition.
One acquaintance of mine used to wear a mask most of the time, including alone in her car. I don’t know why. This was more than two years ago. If it made sense to her, why not?
I don’t think anyone who rides this thing out is going to say, “Dammit, I was too careful. Why didn’t I take more chances with my health and that of my family?”