Because fat and sugar are drugs. People don’t usually think of them like drugs, because of the widely accepted, but very wrong, attitude that only illegal substances can be drugs. Sugar, fat, caffeine, alcohol or tobacco are all drugs. They all trigger a desirable chemical reaction in your brain, and all have addictive potential. At least 2 or 3 of them are also significantly more harmful to you several types of actually illegal drugs.

And in general, though perhaps most strongly for drugs, many people suffer from the cognitive bias of “illegal=bad & legal=good. Automatically and by default”