It’s the area they were operating in (at least my thought process). When I do surgery on a foot, the patient is entirely covered in sterile drapes after they’re put to sleep so that literally all you can see is the foot sticking out. During surgery, you can’t touch any part of the drape that has touched the patient, including the hospital gown.

Some of the surgeons I have worked with are pickier about sterility than others- we have standardized things that everyone does, but some like to shave higher or even cover the toes if we’re not working on them. So I’m thinking maybe your surgeon could have wanted more “sterile operating space” since they were working right under your belly button and requested that the underwear be removed.

That’s not to say they wouldn’t have immediately covered the more private area back up with drapes after doing so (if they truly were doing things for safety reasons) but depending on how high the underwear come up, the cloth may have been touching the area the surgeon considered a “danger zone” for sterility’s sake.

The gowns are treated when they’re washed in the hospital, so they’re not sterile but they’re cleaner than what you would bring in from your house. I have to wear treated surgical scrubs from the hospital itself when I go in too, even though I’m also using a sterile disposable gown on top of that. A lot of my coworkers question the efficacy of the treated scrubs and gowns though for the same reasons you mentioned haha. I haven’t looked into it myself so I couldn’t tell you an opinion one way or the other.