I guess they don't understand that a balanced diet would make you strong so you could fend for yourself and fight the bullies off. Some parents are impossible.

step one get job step two buy your own food

Depending on how old you are, this could be a health issue, or just a social issue. If you are old enough and capable of doing so, get a job and buy balanced food. If you are not old enough, this could be dangerous and maybe you should talk to your doctor. Depending on their health issues as well, as a person struggling to get my own health in order, sometimes having unhealthy food in the fridge causes me to struggle with self control, and sometimes I cave and eat crap I KNOW is bad for me because it is there. You also need to respect their health goals and they may not be able to cope with living healthy life styles when there is constant temptation. If you can get a job, maybe discuss getting a small fridge in your personal space just for your food. Alternatively you could move into your own space and have complete control over your diet and lifestyle. It’s a hard one I know.

Are your parents fat? Is that why that's all they buy?