[...]something as simple as weight loss?"

Weight loss isn't as simple as you think.

Sure it's all about burning more than you eat, but for a lot of people "just eat less and exercise more!" isn't advice they can follow. You seem to have "lucked out" on that front.

The far more interesting and useful question is "what factors determine how easy it is to eat less and exercise more?". This is where it gets nontrivial. You can't even narrow it down to one field of study. I've known people to have success from just changing their diet (not all in the same way) as well as others who have had success from psychological shifts - and one from surgery.

I don't consider LW to be the experts on how to lose weight either, but that doesn't signal incompetence to me. Finding the flaws in the current set of visible "solutions" is much easier than finding your own better solution or even grasping the underlying mechanisms that explain the value and limitations of different approaches. So if you have a group of people who are good at spotting sloppy thinking who spend a few minutes of their day analyzing things for fun, of course you're going to see a very critical literature review rather than a unanimously supported "winner". Even if there were such a thing in the territory waiting to be found (I suspect there isn't), then you wouldn't expect anyone on LW to find it unless they were motivated to really study it.