Saying it's ALL "Calories in and calories out" does have it's limits.
You eat 5000 calories a day in fat and protein with zero carb content... you'll be a concentration camp scarecrow inside of a year. In theory you can make sugars out of non-carbohydrate food stocks, almost all living things on earth can through gluconeogenesis and
some of the sugars
might get converted to triglycerides/fats for storage... and your liver is going to eventually give up on glycogenolysis after roughly 72 hours of such a diet, switching over to ketone bodies instead.
But your GI tract isn't going to come anywhere near to absorbing all 5000 calories of fat and protein.
You're going to poop a lot of it out. And your insulin response is going to grind to a halt as well. And instead lipolysis is going to start up as your body starts attacking fat/lipids to break them up into glyerol sugar-alcohols. And even if you're getting 100% of the lipid calories from dietary fat intake, you're likely to activate the lipase enzymes in your own fat stores which will break them down whether from a caloric intake standpoint you need them or not.
I'm not advocating anyone actually TRY a no-carb diet that extreme, but it AIN'T the fat and protein that's making Americans fat. HFCS, corn, bleached wheat, potatoes, rice, and to a lesser degree sucrose* is doing it.
And enough sucrose to actually make you fat is simply unpleasant or downright uncomfortable to eat. The osmotic pressure on your mouth and stomach from that much sucrose, crystalline, or in suspension is going to make it kind of "burn" just like salt, whereas HFCS is "smooth" and you can just keep gulping it for hours, even if your pancreas started screaming "Uncle!" a long time ago.
The Inuit's with their wintertime diet consisting of almost solely meats caught through fishing/hunting, and other tribal people who's diet was almost all subsistence meat-hunting were thought to be the exception to this, however it turns out they get/got a fair amount of dietary glycogen from eating organ meats of the animals that we normally pass up, and that they actually traded meat and fish to other peoples for plant-based foods. And conversely, many Native north and south American peoples suffer obesity and diabetes at rates even higher than Euro-Americans when they eat a diet high in processed refined carbohydrates.