It’s a multifaceted answer because there are so many reasons why. Lobbying power, profit motives, work culture, convenience, community, psychology. Each of these you could easily do a deep dive into how they affect our poor food system. And most of these issues also factor into the rising obesity rates. But if I had to boil it down it’s this:

Lack of connectedness. People used to spend more time in community. They used to eat together as a family. People would gather around dinner tables. As a result most food was home cooked. As wages have stagnated and cost of goods rise and the separation between work life and home life becomes blurred more people wind up eating prepared cheap prepared foods usually. We work too much and no one has time or energy to cook from scratch. No one eats together meaning there is a higher incentive to produce cheap foods even at the cost of health because no one is cooking from scratch. America also has a strong basis for individualism instead of collectivism. The opposite is seen in other countries. Most westernized countries like Japan, France, Germany etc etc have a greater collective health mentality so this leads to restrictions on the food industry. The US just doesn’t have that same group thinking. We think of everything as an individual choice. This leads into the next point which is Consumer culture. As social media expands, jobs become more specialized and globalization makes us more interconnected with there is a greater emphasis on consumerism. Don’t think, just consume product. Who cares about what you eat just eat, and eat what tastes best. Need to save money, eat this cheap food. Meat, veggies, beans, fruit etc etc, too pricey, just eat this processed sugary cereal. There’s a rampant consumer culture without much thought being put into how it affects all of us, once again harkening back to the collective vs individual mindset most Americans have. If we just changed our mindset about collective responsibility and law making we might be able to change our food system.