Marvel's Reed Richards of the Fantastic Four may be the so-called 'smartest man in the world,' but even he can't cure the Thing and turn him back into his human form. The Fantastic Four's leader has created all manner of devices and inventions, from the famous Fantasticar to teleportation to cures for various diseases and more. Still, his old friend Ben Grimm is doomed to remain a rocky Goliath despite Richards' best efforts, all thanks to his origin story that cements the character.
Jack Kirby and Stan Lee set out to change the superhero genre with the release of Fantastic Four #1 in 1961. Unlike virtually every other superhero from their competition, the team members had no secret identities of any kind (they didn't even bother with masks) and were treated more like celebrities than crimefighting vigilantes. The team was also a family: Reed and Sue were engaged and soon married, and the Human Torch was Sue's younger brother. Ben Grimm was included, but he wasn't a family member in the literal sense - and that's not the only way he stood out from the others.
Early issues of Fantastic Four made it clear that Ben could not control his powers; while Sue could turn visible and Johnny could dissipate his flames, the Thing could not revert to human form. This became a recurring source of drama for the character, would would at times feel like an outcast and a monster who never belonged in society. This occurred in the second-ever issue, in which the team were impersonated by alien Skrulls. "The whole country is hunting for us as though we're four monsters!" says Ben. "Well, maybe they're right! Maybe I am a monster! I look like one - and sometimes I feel like one!"
Ben's greatest stories would often involve him coming to terms with his condition - only for him to sink into depression just a few issues later (as seen in "This Man, This Monster!" in Fantastic Four #51). The real reason why Reed Richards cannot permanently cure the Thing is because his uncontrollable powers are a fundamental part of his character. Turning the Thing human would be akin to permanently bringing Spider-Man's Uncle Ben back to life, or allowing the X-Men's Cyclops to open his eyes without shooting his deadly concussive blasts. The tragedy makes the character, and without it, Ben Grimm isn't nearly as interesting.
It's important to note that even Reed himself shares this viewpoint, or at least a version of him. In Marvel 1602, Richard Reed and his crew became the "Four from the Fantastik" and develop similar powers to the 616 team, albeit 400 years earlier. "Can you restore me to my humanity?" the Thing asks Reed. "I have been a monster for too long." Reed replies "In truth, I do not know, my friend. The natural sciences say yes, a cure is possible. But the laws of story suggest that no cure can last for very long, Benjamin. For in the end, alas, you are so much more interesting and satisfying as you are." So it is with the main version of the Thing, the tragic heart of the Fantastic Four.