According to this paper 'Why Humans Like Junk Food ', there are multiple reasons why people cannot help eating junk foods. I will share the essence from Why Your Brain Craves Junk Food (and What You Can Do About It) The factors are as below:
Feeling when eating
This includes tastes, texture, and falvors. When you eat the food, your mouth and inner skins, nose, and toungue would receive many information. The combination of these are optmized by the snack or beverage comapnies based on million or billions dollars investment over the decades.
The ratio of carb, fat and protein
The ratio of carb, fat and protein is also optimized to make us addictive.
To create the unique but strong sense and perfect ratio, the food companies developed many methodologies. The below are the examples:
Some foods are desinged to experience more than 2 types of different textures. For example, it would be boba tea. Smooth liquid drinks with chewy boba. Another example is pan au chocolat. External chroassent is very cryspy and the internal chocolate is melting. These contrast would make our brains remember the taste.
Acceleration of Salivation reaction
Some ingredients like butter, chocolate, dressing, ice cream, mayo accelerates salivation reaction. That is why people think these foods are tasty.
The food which quickly melt in your mouth is cheating your eating experience. You will feel you eat that food less than actual. So you can eat more.
There are other aspects the food companies try to control our feeling and eating experience. Please check it out.