Fiber slows down the absorption, so there's less spike in blood sugars as when eating candy which is all easy to digest refined sugar.
It’s not automatic, but the processing is guided by whatever processing scales (both in manufacturing and delivery of the “food”) and has been honed by a/b testing and statistically evaluated to be addictive.
Maintaining the complex structure of the unprocessed ingredients isn’t even on the radar.
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.