Human metabolism is a (set of) complex process(es). You are not just burning the plate of food in a furnace.
Of course if you eat less calories than the bare minimum needed to sustain life, you will loose weight no matter what, but that is an unhealthy way of doing it and no sane nutritionist will risk advising you to do so.
On the other hand, it is actually possible to gain weight by restricting calories just a little. Humans did not evolve in an environment where superabundance of food required voluntary abstinence to keep in good shape. When you just cut calorie intake your body enters "starvation mode": metabolism slows down, non essential processes get suspended, you tire out more easily, etc. The result is that if you do not overcompensate with will force and do exercise, you will end up undercutting your calorie usage below your reduced calorie intake, and grow fast.
The body is wise: it knows when things begin to go bad, they always can get worse.
Without agreeing with the OP's point this is a stupid retort. Even with a jet engine you're going to have different amounts of output even with the same amount of input fuel, based on differences in combustion efficiency.
What about other factors? Sure thermodynamics says if you burn more calories then you take in you will lose weight but what about other factor like insulin that cause weight gain? Is eating 300 calories of sugar really the same as 300 calories of fat? I know from personal experience I can eat the same amount of calories on a keto(low carb high fat diet) and lose weight. Although eating fat makes it much easier to cut down on calories and accelerate weight loss even quicker because I'm simply not hungry like I usually am.
There are very few activities that let you meaningfully adjust your metabolism, the effective ones are thermogenic in nature. The only one of these that's at all fun is swimming. I spent all of last year trying to exercise to lose weight, the only thing that worked was lowering my intake.
People say weightlifting works, but it only helps when you're already relatively skinny. I know, I tried. You can be strong and fat. People say running works. It has a small effect but it's nothing to write home about. My sister runs marathons and still struggles with her weight. The problem is that exercise is maybe 10% of the picture, whereas intake is the other 90%. It's easy to fool yourself into thinking you're making progress on your weight goals by exercising, then throwing it all away on extra snacks. People get caught up in chasing mushy health goals and forget about the hard empirical facts.
Of course if you eat less calories than the bare minimum needed to sustain life, you will loose weight no matter what, but that is an unhealthy way of doing it and no sane nutritionist will risk advising you to do so.
On the other hand, it is actually possible to gain weight by restricting calories just a little. Humans did not evolve in an environment where superabundance of food required voluntary abstinence to keep in good shape. When you just cut calorie intake your body enters "starvation mode": metabolism slows down, non essential processes get suspended, you tire out more easily, etc. The result is that if you do not overcompensate with will force and do exercise, you will end up undercutting your calorie usage below your reduced calorie intake, and grow fast.
The body is wise: it knows when things begin to go bad, they always can get worse.