There is a chemical in your brain called dopamine which is an important part of how we feel pleasure. Use of recreational drugs and alcohol causes a rush of this chemical and that is part of the pleasure we feel from using them. The problem is that regular use of such chemicals causes us to have lower levels of dopamine when we are not using them. We end up feeling a desire for the drug or booze to get our dopamine levels back up.

Diets high in sugar, salt, and carbs also causes a dopamine rush. When you eat that food regularly, it lowers your normal dopamine levels, just like drugs and alcohol do, if not to the same level. That is why you feel that craving. Eating such food occasionally is fine, but if you do it to often, you can literally get an addiction to it.

Edit to add -

You mentioned that fast foods are low quality and bad for you. That is true, but but only in high quantities. Fats and sugars were high value foods to our ancient ancestors. Fats are very high source of energy. You get more than twice the energy from a gram of fat than you get from a gram of carbohydrates or protein. Sugars are easily digested making them a source of quick energy compared to other carbs, fats, or proteins. Eating these kinds of foods gave us a survival advantage over those who didn’t, at least until we learned the agricultural skills to make them easy to acquire. Now, many or most people can get such foods any time they want and though they no longer give us an advantage, and eating a lot of them is actually harmful in the long run, those ancient taste preferences still remain in our evolved programing.