This is how metabolic tests work. When you burn fat, it has a lot of carbon in it, speaking generally, after that fat is metabolized, that carbon binds with oxy

This is how metabolic tests work. When you burn fat, it has a lot of carbon in it, speaking generally, after that fat is metabolized, that carbon binds with oxygen and is breathed out as C02. So you can (and people do) use grams in/grams out to determine metabolic weight, but short of a lot of very specialized equipment, it's much easier for someone to have a rough measure of calorie burn at rest by choosing a standard value (or measuring it by the aforementioned metabolic test), and then approximating the amount of calories going in. The psychology/physiology of weight loss is a complicated subject, but the thermodynamics isn't. It's very easy to just say "diet", but the hard part is the motivation, so while everyone is absolutely correct that counting calories is the "optimal" method of weight loss, it's not the strategy that optimizes results across the population.