Doctors have a tendency to blame everything on being fat. This can prevent people from getting adequate health care.
It's complicated, obviously, by the fact that carrying around extra weight is going to exacerbate a lot of existing health problems and create new ones -- but it's not always the only answer, and doctors who refuse to acknowledge that fat is often a contributing factor and not the sole cause of a health problem are doing a real disservice. If nothing else, obesity is neither caused nor cured overnight, and the solution to health problems is not "idk, talk to me when your BMI is in the normal range." Crippling knee pain is not a punishment you deserve and have to just accept for being fat, for fuck's sake. It's a consequence, and one you should do your best to deal with. This means losing weight and trying to find a way to help your primary treatment -- losing the weight -- to be more effective.
Put it this way: poor diet and obesity can cause diabetes. Doctors will tell patients to lose the weight and put them on diabetes medication. They are not going to tell patients to lose the weight and just not prescribe insulin etc, because that's idiotic. It's the same deal with your knees: you are actively treating what is likely the primary cause. That treatment is giving you diminishing returns. Your doctor needs to work with you on this.
It might be helpful to be more firm with your doctor about your expectations. If you went in and expected an instant cure, or for them to disregard the effect obesity has likely had on your injury, then I can see how it could get to a frustrating, unproductive place for both parties. You need to make it clear that you have lost a significant amount of weight, and have every intention of sticking to a plan that works for you for healthy weight loss, but you need some more help to manage your pain and your injury. Chronic, disabling pain is a surefire way to make continued weight loss a hell of a lot harder.
As for people who aren't doctors? Fuck 'em.