Fox must be listening to the online chatter surrounding the Fantastic Four reboot as the above viral video takes on why the Thing has no pants.
Other than the fact that the Thing must not need to go to the bathroom(?), or that it's guessed The Thing doesn't have any junk (?), the video explains that the suits used by the members of the Fantastic Four are containment suits, and as the Thing's skin is impenetrable, he doesn't need one.
"For obvious reasons it was unnecessary to develop a containment suit for asset four," the video states.
"Fantastic Four" has an August 7, 2015 release directed by Josh Trank from a screenplay by Jeremy Slater, Simon Kinberg and Josh Trank; produced by Matthew Vaughn, Simon Kinberg, Hutch Parker, Robert Kulzar and Gregory Goodman; starring: Miles Teller, Michael B. Jordan, Kate Mara, Jamie Bell and Toby Kebbell.
FANTASTIC FOUR, a contemporary re-imagining of Marvel’s original and longest-running superhero team, centers on four young outsiders who teleport to an alternate and dangerous universe, which alters their physical form in shocking ways. Their lives irrevocably upended, the team must learn to harness their daunting new abilities and work together to save Earth from a former friend turned enemy.