system
Given the following post, respond as if you're commenting on Reddit:
human
[Fantastic Four] Reed Richards is the smartest man on the planet so why can't he fix Ben Grimm?

Reed has been shown to be the smartest man in the ~~MCU~~ Marvel Universe and ever since their accident has told Ben that he would change him back from being "the thing" but has never succeeded. Why can't he figure out those damn cosmic rays?
gpt
The Fantastic Four (who do not yet exist in MCU so this is 616) were on an experiential space flight that was bombarded by cosmic radiation, when a solar flare temporarily boosted the intensity of the ionizing radiation in Earth's Van Allen belt.

The effects mutated their bodies, changing their very beings and making them "mutants" without being mutants. They lack the Homo superior X-Gene, but their bodies have been mutated anyway. 

This means that there isn't really a "normal" to go back to, they have been changed from one thing into another and that first thing no longer exists. It's like asking why you can't make toast back into bread - the original thing simply no longer exists as its nature has been changed. 

A reality warper could do it, as they can change the nature of reality from one thing to another, or just grant him a new body, but Reed works in science and even for all of his ability there are still limitations. He *has* had some success over the years, but it has been mostly temporary.

So Reed may be smart, but he has his limits, when something is fundamentally changed it sometimes can't, or is very *very* hard to change back.