It has to do with how the body responds to the release of insulin. Keto/low carb works because your blood sugar stays low, so fats (generally) become fuel rather than storing it away in your arteries.

For a more elegant and educated explanation, I highly suggest Gary Taubes: Good Calories Bad Calories (or any of his works, really).

Edit: I should add that for people without metabolic disorders, there is more wiggle room, but the principle stays true.

Building on the response from u/the—larch:

Carbs and fats are processed by two different metabolic pathways and the carb pathway is more efficient so it is used first. You can then burn up the glycogen provided by the carbs while your body is slowly converting the fats which will feel like a blood sugar crash, so you eat more carb+fats. The process keeps going through the day and the fat has no where else to go except as triglycerides in your fat cells.

This issue can be abated if you’re willing to and can ignore the carb crash as a signal of hunger or if your carb source has high fiber as part of its nutritional matrix because the fiber slows down glycolysis. I would imagine we could potentially one day design a food with the ideal matrix where the ratio of carbs/fiber/fats provides the right amount of energy at the right time, but it won’t be as tasty as a donut.