I believe it was Spielberg that said he felt cheated (as a youngster)
after the movie, not seeing what the Krell really looked like, since
the film teased him with the doorway architecture. "What did the
creature look like that would need that shape of a doorway?" A
trapezoid, iirc.
Sometimes "not seeing" is better than "seeing". Not seeing the Krell leaves them as mysterious beings. There are many movies where "not seeing" is in some ways the key to the movie. In "Alien", the alien creature is almost never shown except for a few seconds (Ridley Scott was afraid the actor-in-a-costume would become obvious). In "Jaws" the shark made few appearances (due, apparently to technical difficulties), but that worked to the movie's advantage.