It is a fact that too many calories compared to exertion makes you fat.

It is a fact that fat is calorie dense.

It is a fact people need fat in their diet for proper skin, hair and nails as well as thyroid function etc.

Experts agree about 30% of our daily calories should come from fat.

What does an average person need for calories? It depends on how active they are.

A person with a desk job that moves very little during the day and doesn't exercise only needs like 1600-2000 calories a day, or else they gain weight.

A double cheeseburger with medium fries and a drink has over 1100 calories.

A piece of chocolate cake with frosting has something like 400 calories.

See a problem? Who is content with that for a day?

Switch the same person to a manual labor type job and the calories needed jump too.

To advertise that fat doesn't make you fat is as untrue as saying it is the only thing that makes you fat. Too many calories from any source will make you fat.