A big part of it is the supersaturated foods we have now are far, far more sweet/salty/fatty than our tastebuds evolved with. Sugar fat and salt taste good to u

A big part of it is the supersaturated foods we have now are far, far more sweet/salty/fatty than our tastebuds evolved with. Sugar fat and salt taste good to us because our body needs them and in ancient times the tastiest foods available were also the healthiest (fruits, nuts, meat, etc). Evolutionarily speaking, food tastes good to encourage us to eat it, and it makes us happy to eat good food because that means we're keeping our bodies alive/healthy. Nowadays, we can make food that has far more sugar/fat/salt than our tastebuds evolved with, so the tastiest foods are now the worse foods for us, but the biological pressure to eat them hasn't gone away. To make it even worse, things that make us feel good are things we can get in the habit of doing (i.e. addicted) very easily. To make it even worse, you have to eat. If you're addicted to marijuana (assuming it's purely psychological addiction here) you don't have to smoke a less fulfilling joint every day (reminding you of the more fulfilling joint you could be smoking), you can just quit cold turkey (which is also hard but less so). A lot of obese people are literally addicted to unhealthy food since it provides so much more stimulus than our brains were evolved to handle. And to make it even worse, being obese tends to lead to low self-esteem and a common way to assuage self-esteem issues is to engage in pleasurable activity... like eating more unhealthy food. It's a vicious cycle of eating poorly, getting fat, then eating poorly to comfort oneself, getting fatter, eating more, etc. It can be very difficult to break the cycle, both because of the super-stimulatory nature of the food and because it's impossible to just quit eating cold turkey. Every healthy meal is a reminder of the addiction you could be feeding and with the high availability of poor food (especially in the US), it's even harder to stay healthy. In short, we are of wired to find bad food comforting in a roundabout way, and the problem is far harder than you give it credit for.