The food manufacturers make it very easy to overeat, by providing cheap low-quality food that is full of sugar.
I’ve seen myself the issue that, if I buy groceries based on what I feel like, I tend to end up buying premade food.
So, I consciously avoid that. If I have no frozen pizza, no chips, no peanuts, I can’t eat them either. And I’m too lazy to go to the store right now to buy those, so I instead end up making a healthy meal.
The "but it’s healthy!" ads don’t make it easier, as they try to trick your mind, too.
Honestly, if everyone would have the willpower to only buy what is good for them, the food market and the ad business would not be able to exist anymore.
Coca Cola would be bought almost never, except maybe for parties or during holidays or on birthdays (like my parents used to do when we were kids – softdrinks only on these special occasions).
But it would also affect a lot of the "healthy" food that isn’t actually healthy, like "low fat" yoghurt (20% sugar content), or "fruit juice" (with 10% added sugar), etc.
I think @kuschku got the (slightly tongue-in-cheek) intent behind my original comment, but you're right, and the choice/willpower problem is there, and it ranges from difficult to impossible to overcome for many people!
This is actually a big part of why the food industry's actions putting sugar and fat in everything in order to make it sell more, regardless of the health effects, is being seen more and more as bordering on malicious and being considered for oversight and regulation. Food manufacturers, as businesses, should be doing whatever they can to sell more, but as providers of basic human needs, their sales tactics are now actively working against our interests. Our physiologies are built to take more food than necessary when its available, so these perfectly engineered high calorie bliss-point foods make willpower harder to fight than ever before!