Why do you think the doctors advise things, or want to give you tests or review you? They are snowed under with patients and red tape, don't you think they would be happier not bothering? Believe it or not, they do it because their job is to make you healthier and they are honour bound not to do anything that makes you worse. They don't want to take over your life, but when you go to them for help, they must consider all your ailments and conditions.
Every medical intervention has an effect, some effects are good and some bad. If they give you "this, that or the other" without reviewing ALL the effects it has on you, it could be that some other condition is getting worse.
You say yourself that you have multiple health problems. That must make it more difficult for them to prescribe treatment that will help one thing without affecting something else, and your weight is one thing that can have a lot of impact on everything else and also, most importantly, it is something that YOU can do a lot to improve.
If you don't want treatment, you can refuse it, but surely you can see that if you keep refusing to do something to contribute to your own body maintenance, you can't expect someone else to produce a magic wand from their medicine cupboard that will cure you with no side effects?