I think people do care about what they eat, which is exactly why some people with weight problems are drawn to unhealthier foods like the example you give, because they prefer the taste of those sorts of foods over healthier choices. I think what people don’t immediately care about is the consequences of their food choices. I think that if people didn’t care about what they ate, then unhealthier foods wouldn’t really have such a draw. For many people that seem to be “hooked” on junk food, the idea of healthy things like fruit and vegetables is not something they enjoy or are accustomed to. If those people are just thinking about what satisfies them in the short term, then I can see why they would choose to eat unhealthy things that they like the taste of.
I think the answer to this question lies in the choice between immediate versus delayed gratification. Someone might find an unhealthy food item instantly gratifying, immediately providing them with pleasurable sensations and satisfaction. The immediacy of the pleasures of junk food, for some, may override the delayed gratification you get from eating well, and thus experiencing better physical and mental health in the long term. If people don’t think about the future, in the often very brief decisions they make in what food items to choose, then it’s understandable how people are drawn to items that might give them immediate pleasure, even if another item would give them a greater benefit in the long term.