Addiction to sweet stuff. Due to chemistry, a can of cola contains a lot more sugar than a person would normally want to consume from that much liquid. The acids sourness apparently balances out the sweet while adding a bit of tang. Then again, due to chemistry, sugar(soda, ice cream, coldstones 2k calorie shake) and carbs(deep fried twinkies) are more quickly digested than protein and fat. This is primarily why Keto and Mediterranean diets or diets that are very similar(high protein, low carb/sugar) function so well for keeping in shape. Combine this with a poor level of physical activity and beasts just fatten up. Basically, bad health choices constantly, despite all the information available to them including their own body telling them every time they try physical activity, like walking to the toilet for the 10th time that day.