Decades ago, about 30% of the people who had celiac disease would waste away and die, especially children. Until a doctor noticed that if the people ate 12 bananas a day they seemed to do better. Of course, 12 bananas a day is a kooky diet, but people who eat 12 bananas a day tend to eat less of everything else, including less bread and gluten and so they would do better. Later science figured out bread was a problem and then eventually identified the specific protein (gluten) that effects people with celiac disease.
I think there are other foods that bother different people, but we haven't managed pin down exactly which foods bother which people. It's a hard problem, but surely there are other cases of "if you just avoid this specific food ingredient, you will recover", just like celiac disease.
So, on the one hand, sharing advice about which diets worked and didn't work can be helpful (like the 12 bananas a day diet). On the other hand, "I tried an unusual diet, which also happens to have a good amount of marketing behind it, and things are better" is one of the most common bits of advice you'll find for many diseases, and the problem is everyone recommends a different diet.
A relative of mine has Chrons disease and was close to death before it was diagnosed, the doctors recommended a junk food diet basically, high calorie foods with almost no fiber, and that's how he eats now and is doing much better. I don't recommend everyone follow the same diet, but for whatever it's worth, eating junk food has helped my relative, true story.
And this is what I mean, there's so much conflicting advice about which diet to try. I can't fault people for trying different diets, because I believe there are many unknown disease, like celiac disease, which can be treated with an exact (but as of yet unknown) diet. I also don't fault people for giving up and not enthusiastically trying every diet-of-the-week that gets suggested, it's tiring. Also, it's notable that the diets with the most marketing tend to be the ones most recommended.