Laziness. Supermarkets still sell raw food (vegetables, meat, etc) that can be used for cooking. But it takes time and effort to do it (+washing) so buying already made but unhealthy dishes is appealing for a lot of people.
Edit - https://www.consumerreports.org/food-shopping/supermarket-pr... The article doesn't call out higher amounts of fat in prepared food, but does mention higher cost and added preservatives.
I suspect it would be very difficult (ie expensive) to make ready made meals anywhere near as appealing as home cooked without using these tricks.