I have observed that myself, and I honestly don't know. Maybe it's just that they're so busy they don't have time to take care of themselves? I assume it's a case of the cobbler's kids going without shoes....
Two of my neighbors growing up were OR nurses and each had to be at least 150 lbs over weight. And when we played cards at night they always talked about their feet hurting from all the "miles" they walked that day. And when ever I am in a hospital for any reason I take mental notes, and it seems to be running about 85% of them are overweight or WAY overweight. Maybe even higher.
And how come they are never hot and scantily clad like in the movies?