https://www.fourmilab.ch/hackdiet/e4/
The author's theory is that the issue is not on of metabolism, but that people overeating do not have a good "I'm full" mechanism.
Most people like you that "can eat all they want and not gain weight" in reality... don't. They balance out their intake naturally, if not in the scope of a meal, over a day or so.
For example the concept of leaving unfinished food on the plate it completely alien to me (or not going for seconds if available), while most normal people just stop when they feel they are ok.
In my very personal and anecdotal experience every time I started dieting counting calories and exercising I have always lost weight, consistently, while when I stopped I gained weight back (again, consistently and predictably).
Most other diets that have a scientific basis should be efficient as well, since what they all have in common is to enforce some sort of control on your intake.
Either way I agree that it's about genetics and willpower but, not having the possibility to change genetics, the mechanism to counter it exists and it's not a mystery, it's really a psychological issue, IMHO.