Processed carbs are easy to eat and fill you up quickly, but not for long. Excess carbs are converted to sugar and stored in the body as fat, starting with visceral fat. Generally people just eat too many.
Keto is about training your body to use consumed fat as fuel rather than storing it, meanwhile reducing your fat/energy stores(blubber).
Its a whole lot healthier than eating crap and becoming obese.
Not all calories are equal. If you eat enormous amounts of green vegetables and nothing else you will not get fat.

Once again balance is the healthiest but not all can maintain balance.