Your comment ignores:
>What you eat matters
>The state of one's digestive system matters
>One's overall nutritional gaps matter
>Activity level matters
Yeah... It's an equation with two sides, caloric intake and caloric output. If you take two people, and one of them is sitting there posting on Twitter all day and the other one is riding in the tour de france, it should be self-evident that if they are eating the same amount the outcome will be significantly different.
So let's take another situation, two people sitting on Twitter all day, but the basal metabolic rate is slightly higher in one than the other. We know that certain things affect the basal metabolic rate such as general activity levels or genetics, and potentially certain foods may it affect that as well.
The composition of the food can also make a big difference. As I recall, the test for caloric intake of food basically involves burning the food in a controlled manner. Well there are things that are highly calorie dense but also somewhat inedible. For example, a piece of wood is made out of all kinds of carbon that will burn, but it is insoluble fiber when it comes to our body's ability to break down and use those calories, and so it will basically leave the body the same way that it entered. If you have one person eating 2,000 calories of wood and another person eating 2,000 calories of sugar the body is definitely going to be interacting with those two things in a fundamentally different way, and so one person will starve and the other person will be more or less just fine.
The nutrients contained in food can also have a big difference. When I first started taking vitamin b complex, I was shocked I just how much energy he gave me, because vitamin B is critical in helping your body process fat.
All of this is true, but it's important when trying to lose weight not to get bogged down in details -- in that case calories are the easiest way to gauge where you are and where you're likely to go, but we're not talking about individual weight loss here but the populations, and given the date points we're looking at the details start to matter.