I think they come up with “New” diets for various reasons
1, Money to sell them & diet books
2, The New is Better Principle
3, Pandering to the wants (stupidity) of Women & Fat lazy people
4, It’s The Easy Diet People are looking for that Magical one
For Sure some diets have results.
Simple fact is Move more & eat less works - only Many don’t want to move more or do anything
Other than sit at work / home / Car & eat Then Complain about being / getting fat.
There are diet plans that work if people follow them and learn to eat differently from them. If however, they diet and then go back to eating the way that made them overweight in the first place, the diet is blamed. Overly strict diets are less likely to work because they are meant to be temporary. Weight Watchers is actually a good diet, if people follow it indefinitely which shouldn't be hard to do since nothing is forbidden, just the quantities are controlled so that you don't overeat, which is the basic problem most people have with food. Also, Weight Watchers offers a balanced diet. Most Diets that omit a food group, such as carbs or fats are not going to work in the long run because they cannot be sustained indefinitely.
Poor eating habits begin in childhood, IMO. My mom fixed just enough food for the three of us when I was growing up. A serving of vegetables, a portion of meat and usually a starch or a second vegetable. Desert was a rarity. There were no seconds or help yourself to as much as you want from serving bowls. The only exception was at holiday meals like Thanksgiving and then only if we had company. There was always fresh fruit in the house if you wanted a snack. Chips and such were reserved for weekend big barbecues with lots of people. My stepdad was a "meat and potatoes" person. He wouldn't eat casseroles of any kind. When we had pizza, we each ate two slices and maybe some salad.
A lot of parents today buy all manner of crap food and let their kids have at it. Families eat out much more and most restaurants give you way more food than you need. Take out is very common. Often meals are from fast food joints which have an array of bad choices and few good ones, diet-wise. Who the heck knows what is in that 'secret sauce' they smother your hamburger with. French fries are a mainstay at fast food restaurants. Try and get a healthy drink such as iced tea or real fruit juice, they are hard to find. Mostly all that is offered is a ginormous container of pop. I get water. The cup they give you is so small that you'd have to fill it three times to make one glass full.