This method is simple, and works well enough for enough people, but it doesn't for some. I still kind of agree with you, because I have a sneaking suspicion that people mostly cluster into groups of "any diet / intervention works" and "none of them work".
Unfortunately, the body itself is a complex system, and likes to throw curve-balls. Going by some of the things I've read over the years, it's clear the body has defense mechanisms against starvation - such as lowering metabolism and activity levels in lock step with diminished caloric input, or turning you into food-obsessed zombie, hardly capable of a non-food-related thought, until you feed the body enough to cancel red alert. Problem is, for some people, those defense mechanisms seem to kick in before they reach negative energy balance. As simple as calorie counting is, it's not helpful if the body hits the "override" switch before you reach Calories In I don't know if there's a solution for this case, but I suspect all the variety of diets is really just people doing a blind search in hopes of finding specific tricks that shut down the defense mechanism in subgroups of individuals.
(In a sense it's like a puzzle game: your goal is to sneak past a security system without raising an alarm. The security system consists of a great variety of sensors of all kinds - but only a random subset of them are in use, configured with random thresholds. Every player gets their own, unique puzzle.)