Processing is by definition used to extra more of something (usually calories) than would otherwise be present. It doesn't have to be bad, but it's definitely something that doesn't occur naturally and so we may not be well adapted to it. Thus, the chance of it being "worse", health-wise, than the natural thing is higher.
Processing almost invariably means taking out and throwing away the fiber. But the fiber plays an absolutely vital role. Leaving the fiber in is so anomalous they have to note it in big print on the label, e.g. "whole grain".
There are very serious reasons to treat the fiber as deeply precious, and removing it as deeply harmful.
