Probably because they are knowingly exposed to infected people with high viral loads who are sneezing and coughing a lot, and spreading droplets and aerosols everywhere.
You really don’t encounter that in casual everyday life where you are in your house most of the time and going to the store for a few minutes every couple of days.
There’s a big difference between everyday life and medical professionals.
I’d argue workplaces with poor ventilation, crowded work stations and shared bathrooms will probably spread this quite efficiently.
Before they got serious around here, my last trip to grocery store had 4 coughing people in it. Those coughs travel two aisles away. Now they would probably kick your butt out if you were coughing excessively.