Doctors used to believe in leeches and bloodletting. They just follow whatever the mainstream is.
All the research actually in the field is showing more and more that human adiposity is correlated with some ill-health effects but not causatively linked.
Research also shows that weight-loss interventions are very rarely successful, and even the most optimistic weight-loss intervention leads to, at best, the loss of a few pounds over long terms.
And, again, we are using weight as a totally arbitrary marker for health with very little real evidence that it is in any way meaningful. We know that at real extremes there are health hazards posed by high adiposity and low adiposity, but most people fall into a perfectly healthy weight, even if their BMI classifies them as obese. Of course, BMI categories are also totally arbitrary, it's basically the IQ of dietetics.
There is so much bunk science in dietetics, it's unbelievable. It's like the modern version of scientific racism. In a couple of centuries I imagine humans will look back on us like "how did you ever believe this shit"