Here are some reasons why he might choose to appear in some movies:
* Fun and playful: King is known for having a good sense of humor and enjoys being involved in the adaptations of his work. He might see a cameo as a fun way to interact with his fans and the creative process.
* Support for the film: He might appear to show his support for the director, the cast, or the adaptation itself.
* To make a point: In some instances, he might have a specific reason for appearing, such as making a small joke or emphasizing a particular element of the story.
However, there are also reasons why he might not appear in every film:
* Focus on the story: He wants the focus to be on the story and the characters, and his presence might detract from that.
* Availability: He may not always be available to film a cameo due to his busy writing schedule and other commitments.
* Preference: He might simply not be interested in appearing in every film based on his work.
Ultimately, it's up to Stephen King himself to decide whether or not he wants to appear in a particular movie. His appearances are often a delightful surprise for fans, but they are never a guarantee.