There are so many reasons, like rampant rape, the near requirement in some prisons to join a gang to not get beaten up, $10 for a package of ramen in the shop, routine abuse of things like "solitary" by guards to harm people for fun, $3 a minute to call your family, and horrific situations where the warden is made responsible for planning food, and also given the rule that whatever they don't spend of the meal budget they get to personally keep, which results in inmates eating a slice of american cheese on two slices of bread twice a day for most days.

The punishment of prison should be entirely limited to losing your independence and the dignity that comes with it. It should literally be like childhood "time out". These are still human beings and we owe it to them to treat them like people, they just need to be silo'd off from society for a bit sometimes.