I’ve heard it explained pretty decently.
The body can only store about 2000 calories worth of carbs. If you rely on carbs as a fuel source, once you’re out: you’re out. You gotta refuel. It’s why people “hit the wall” in a marathon. They just plain run out of fuel.
The body, meanwhile, can store hundreds of thousands of calories of fat. We have near limitless potential to do so (Hello “My 600lb life”). You run off THAT fuel source, and you can go for a LONG time.