A person needs about 2000 calories a day. It is extremely hard to get that much from fresh produce. You will need to cook something, and that takes may more tine than microwaving a hot pocket.
It is actually easy to get 2000 calories a day on cheap ingredients from fresh produce. It has to do more with dietary habits and convenience than anything else. A cup of beans and a cup of rice can cost less than a hamburger and provide you more than 800 calories while not creating long term problems for your body. A $50 instapot, and you don't even have to actively cook. But well, hamburgers are tasty because of all that salt and sugar, while rice and beans aren't.
Right, but you never eat only "Fresh Produce", and you shouldn't either. If you look at Macro nutrients, fresh produce usually isn't the biggest part of your calorie intake unless you include potatoes (they last pretty long tbh). Fresh produce usually provides you other nutrients. I was simply suggesting that there are alternatives to processed food that is equally cheap and accessible, but we just don't want it.
Make sure you soak beans before you put them in the slow cooker. Or else you're poisoning yourself! Uncooked beans contain bad proteins because they are legumes. You either need high heat or to do soak them and pour off the water.