I think weight loss is elusive in general because there is not as much of a biological imperative to being thin as there is to carrying more body fat. In terms of survival and even thriving, there is possibly little lost in carrying an (supposed) “extra” 20-40 pounds and possibly everything to gain. But I am totally open to being wrong on this.
Also I think our bodies have long memories.
As for people who are severely overweight, I do consider that a disease of sorts, a dysfunction brought on by lifestyle/genetic factors. Repeated dieting over a lifetime does seem to create a body that tends to obesity.