I suspect it's related to the "default" diet that we consume. You can be healthy by eating a healthy diet, but that takes a conscious effort and we know that the food that's most pushed in modern western countries is full of sugars and carbohydrates. Most people eat the default diet, and for the modern person that might be microwave meals or takeout. Presumably in previous generations the default diet was more healthy, because people had to cook due to the lack of quick-and-easy options like oven-ready pizza and chips.

Overall it seems like a fundamental issue with the "food ecosystem" we surround ourselves with, and the types of food that it encourages us to eat. You could even include in that the inclination to treat eating as a chore to be performed instead of cooking and food as part and parcel of a fulfilled life, leading to products like soylent that promote themselves as a least-inconvenient meal.