Why people have to eat so much?

Lots of reasons: For the first time in history, most people have access to more than enough to eat most of the time. There's something in us that tells us to store up in case of famine. Our rational minds can overcome this but often don't. People also eat for reasons other than hunger -- especially for comfort. Best is to learn to choose non-greasy, non-sugary, non-salty stuff -- nobody got fat on raw peaches.