Sedentary lifestyles are a huge factor too.

Many elite athletes devour ultraprocessed foods. Elite cyclists have pushed their performance limits by eating as much sugar as their digestive systems can handle. They eat sugar until they puke and it trains their guts to tolerate more sugar in the future.

Sugar and other simple carbs are quick burning fuel. You don't want to be flooding your blood stream with fuel when your body doesn't need it, but when it does, well, it's fuel, it gets into your system fast, and it makes you go.