King Arthur: Legend of the Sword is hoping that summer movie audiences have a hankering for a blockbuster Medieval fantasy epic. Probably not since the last Hobbit movie has there been a film of this scale dealing with knights and monsters, so you'd think the movie would be pulling out all the usual summer blockbuster stops to get fans psyched for the start of a potential new franchise. And yet, a King Arthur post-credits scene doesn't exist, which is a bit surprising. Why wouldn't the movie, which is clearly looking to start a new big budget series, feature a tease for a future installment at the end of the film like virtually every other movie it's competing against?
There are several possible reasons as to why there is no post-credits scene after the film. One could be that, with the exception of Marvel, many other big franchises are no longer including them. Even franchises that used to rely on them, like Fast & Furious, seem to have given up the practice. Another reason could be that the film teases all it wants to in its regular runtime and ends in a satisfying place, leaving any extra scene redundant. But the most probable answer is probably that there is so much that is still undecided regarding the future of this would-be franchise.
For one, the movie has had a bit of a troubled journey to cinemas. Production began in 2014, and the film was slated for a July 22, 2016 release. This date was then postponed until February 17, 2017, then pushed back again to March 24, 2017 before finally settling into its final release date of May 12, 2017. This sandwiches the film between some stiff competition in the action genre, with Guardians of the Galaxy: Volume 2 having opened the previous weekend and Alien: Covenant arriving the following weekend.
Yet that doesn't mean the movie might not still do well, or that it can't spawn sequels. An early plan from Warner Bros had the film starting a cinematic universe filled with movies based on different characters of Arthurian legend, and the latest plan from late 2014 had the movie kicking off a six-film franchise. It's not clear if this is still the goal or not, and will likely depend upon how well the movie performs at the box office, but the lack of a post-credits scene may indicate that the studio isn't sure if the movie will spawn a sequel or not and is waiting to see what happens.
But that's just speculation. It's hard to pin down the true reason as to why there is no post-credits scene after King Arthur: Legend of the Sword, and all I can say for sure is that there isn't one. So just enjoy the movie for what it is, and try not to worry too much about whether a sequel is coming or not.