Any idea why that might be?
The only thing I can think of is the relatively unfettered access to some highly addictive controlled substances. These substances are very useful for patients undergoing surgery, but they do not have access to them outside the OR.
I remember in residency that there was some testimony from prior anesthesiology practitioners who were reformed addicts who warned about the highly addictive properties of newer anesthetics such as Fentanyl, Sufentanil etc.
Other than that, I do not know...
"Any idea why that might be?"
I have heard that the anesthesiologist is the most important person in the operating room since he/she literally holds the patient's life in his/her hands for the duration of the operation. Maybe that awesome responsibility gets to them after a while. The operation could be a complete success, yet the patient could end up dead because of something the anesthesiologist did wrong.
Just my two cents.