Apart from the cheap, unhealthy ingredients and slick marketing, modern processed food is also engineered to be appealing; and for the want of a better word, addictive. Prehistoric man would have to do a lot of hunting and gathering before he would consume the equivalent amount of carbs, sugar and salt that one could quickly and easily consume in a single meal from a fast-food restaurant. Humans never evolved to be able to cope with the amount of crap in modern food.

We also evolved to hunt or farm our food before we could eat. Now with the internet, you can order your groceries with the barest physical exertion - the click of a mouse.