One of the best reasons to avoid eating stuff in boxes that resembles food – are you entirely sure that what you’re getting is what the list of ingredients says is in the box?
If you buy an apple, that’s an apple. No doubt about it. Carrot? It’s a carrot. Box of Velveeta Cheesy Skillets Singles? Maybe it’s made with durum wheat pasta, and maybe that cheese-resembling goop is made out the ingredients they claim are inside, or maybe it’s made with something else, either out of laziness on the part of the manufacturer, stupidity on the part of someone in the factory or downright malfeasance driven by profit motive.
The reality of the very, very broken US food system is that progressive waves of government deregulation have resulted in very little of the food you eat being inspected for quality control. And even when inspections do happen, the inspectors have little authority to punish manufacturers for wrongdoing.
I worked for almost 10 years in the pharmaceutical industry, and we were held to exceptionally rigorous standards in our manufacturing processes… although not so much by the US government as by the regulatory bodies that licensed the sale of our products in Europe and Asia. I’ll put it this way, we had a plant shut down by inspectors… from England. The FDA came in afterward and agreed with their findings. But packaged and processes American pseudo-foods aren’t usually sold overseas (because our cousins across the pond aren’t actually interested much in eating Velveeta Cheesy Skillets Singles, unless they’re here on vacation and very drunk), so the meager inspection regime of the FDA is all that stands between you and ingredients that really don’t belong in that box, but are there because someone is trying to pad their profits.
Now I’m not an unreasonable food fascist. Sometimes we’re all going to eat junk, but if you expect to be healthy and maintain that health long term you need to make sure that the vast majority of the things you chew and swallow are actual food. And if you’ve got food sensitivities, well, probably shouldn’t buy anything that comes in a box. It could, quite literally, be almost anything.