A number of reasons. The whole “well just wash” thing kind of presumes a certain quality of life and access to hygiene that isn’t guaranteed–my doctor’s opinion on this was informed by his experience in the jungle in Vietnam; it is more hygienic that washing, as obviously soap and water doesn’t remove all bacteria and fungus that live on the skin, especially in folds or crevices; it’s a surface that goes right over the top of an open orifice, so the risk of infection really isn’t something to leave to chance as to whether or not you’ll finish the day with a hot shower and soap; adult circumcision requires open wound care protocol, which carries its own risk of infection, in a very sensitive area, prone to healing complications, whereas babies have veritable superhuman healing powers and it heals in a few days with Vaseline; as a baby it’s a basically an outpatient dermatological procedure, as an adult it’s an actual surgery, with much greater risks of complications. It’s a no brainer; it was recommended by every doctor we asked, who all considered it a no brainer. I know you can find anything online but the science and weighing of risks on this is makes the decision pretty clear.
E: Dumb, anti-acience people are downvoting this. Apparently they know more than doctors.