This is a myth.
When you actually track what these people eat, their daily caloric intake averages out to numbers you'd expect. At best, the difference between "fast metabolism" and "slow metabolism" people in studies comes out to a couple hundred calories per day.
You may witness someone consume 2 pizzas and a soda in one sitting without getting fat, but that person is consuming fewer calories for the rest of the day or the week to offset it.
Some people just only want to eat an amount that doesn't get them fat. They aren't using any willpower or anything, they just don't want to eat more than a healthy amount for them.
I am not one of those people (I am overweight when I eat as much as I want), but I am for other things. For example, I don't drink very often (only maybe 10 times a year) and don't get very drunk when I do. I just don't feel a desire to drink very often. It takes zero self discipline to not drink for an extended period of time because I don't have the desire for it.
On the other hand, many alcoholics struggle mightily to avoid drinking. It takes all their willpower to not drink.
Everyone is different in their ability to avoid different addictions.
So, metabolism is really just an expression of how active we are. Some athletes eat 5-10,000 calories a day just to maintain their weight. Hard working laborers also need a lot more calories than office workers.
Meanwhile, people blame their metabolism as if it's just a trait they can't control like their height. You absolutely can increase your metabolism, just move more. This is also likely a big reason why many people gain weight in adulthood - in school we have weekly gym classes and we spend lots of time running around ad playing with friends etc. Then we grow up and our activity level plummets, but we don't adjust our diet to accommodate our much lower metabolism.
If you construct your argument to exclude the 2 out of 3 people who are not overweight and then point out that the remaining 1/3 is overweight, that's the definition of confirmation bias.
It's also a disingenuous argument because not all of those 40% are actively trying and failing to modulate their weight in any way. Most people just don't care.
For example: weightlifters. It’s standard operating procedure to “bulk up” - periods where lifters overeat to promote muscle growth - and then diet to “cut” weight to drop the fat gained during the bulking period while keeping the muscle. They tend to be extremely motivated individuals though.
This is a touchy topic, but I would like to point out that you're missing the obvious confirmation bias that comes with this observation.
There are many people who modulate their weight by changing what they eat, how much they eat, reducing snacking, meal planning, and changing their shopping habits.
You don't see them among the millions of people failing to lose weight or paying for expensive solutions because they quietly solve their problem.
I'm also not suggesting it's easy, but we should acknowledge that many people do successfully control and modulate their weight through dietary and habit changes. There's a survivorship bias problem that occurs when you only look at the remaining sub-group who has the most difficulty with this.