1) Ensures that any surface or airborne pathogen generated the evening prior is likely dead.
2) Less people creating respiratory borne aerosols.
3) Intensity of exercise is probably lower, thereby spreading less aerosols, given most people do not have peak performance early morning.
4) Someone who is sick or is recovering from illness is unlikely to get to the gym early.
5) Perhaps skip the gym during peak flu and cold season.
For decades I would get sick 3 to 4 times a year but now I haven't been sick in 28 months (personal best by 2x) since becoming a pandemic hermit. I wonder how many of my colds/flu's were contracted at the gym and have hesitated returning but miss this part of my life.