Many doctors have side hustles. I believe that it is more common now than 20 years ago when I got into medicine. A colleague of mine had 2 side hustles within a year of completing residency. They are providing him safety from his main practice if things were to change abruptly.
I would remind you that medical education and training is a grueling process such that most graduates want to earn money in their field as soon as possible. Usually, a side hustle is FAR from their minds. However, the business of medicine is a constantly evolving environment leading to a very high burnout rate. A side hustle quickly becomes an escape plan.
The notion that many surgeons are too busy to have side hustles is not true. Many surgeons have side hustles that pay in excess of their practices. Go to the CMS Open Payments website and search physicians in your area or just list them by the payments they received from certain side hustles. Sometimes, I find it hard not to be jealous.