People don't necessarily buy unhealthy food because it tastes good (of course we all do time to time), but because they can make a lot of it quickly and for very cheap. And for most folks, am nintenho, they simply do not have the time or budget to eat healthy.
It doesn't even have to do with caloric intake, because eating right will always make you feel better. That doesn't even make any sense. Junk food is designed to make you feel not full.
To buy healthy produce and keep costs in line, that requires the person to do most of the prep work. That requires time.
Prepared meals of the healthy variety are significantly more expensive then unhealthy, processed foods.
If you're a single parent or someone with little money, who works an unreasonable amount of hours per week, are you going to spend a certain budgeted amount on skinless chicken breasts, spice/marinate them and grill them or buy a box of fried chicken you can microwave?
Your ability to not comprehend that is mind boggling.