Because they don’t control their portions or eating habits. Take a look at the UK TV show “Secret Eaters”* which sets up secret cameras in and has investigators follow a different household every episode to estimate everything they eat down to the tablespoon of sauce. It’s an eye opening look into psychology. Lots of people think they’re eating 2K kcal or less but are in reality consuming 3-5K per day.

* full episodes on Youtube