Note that the issue is that there is a human in that system.
The problem is that we (rightfully!) don't easily allow experiments on humans that might be harmful.
And, as an aside, nutrition and weight management does have people doing real science (things like studies of hunter gatherer societies with deuterated water), it's just that the results aren't very flashy (ie. a human burns roughly the same amount of calories regardless of activity level--so the only way to lose weight is to eat less--moving more is healthy but doesn't contribute substantially to weight loss).
Here is an interesting Stanford Medicine study of individual variation on the impact on low carb vs low fat. The punchline is that they still don't know after a fairly rigorous study.
The entire area of nutrition and weight management is a joke. Medicine is problematic and Psychology is questionable.
Another problem is that the popular media publishes inconclusive studies because they are click bait. People read them because they are interested in living longer and losing weight and being happy.
Fear also turns studies into clickbait. The latest was the blizzard of articles on if bugs are dying off or not.
Then there is the fact that negative studies tend to be ignored and when the study failed or it introduces data that does not conform to general knowledge.
All this makes people question studies where the data is conclusive.