The Dark Tower looks to be the start of the next major epic fantasy franchise. Like Harry Potter and The Lord of the Rings before it, the film is based on a popular book series, and brings an already dedicated fanbase to the theater. And with eight books and numerous canon comic books from which to draw material, the film series could very well end up being just as lengthy and robust as Harry Potter. So why is it that The Dark Tower post-credits scene doesn't exist?
There are a few likely reasons why The Dark Tower eschews an extra tag at the end of the film, but the biggest one probably has to do with the franchise's bizarre set up. Unlike LOTR or Harry Potter, the movie is not based on the first book in its series. Instead, it's technically a sequel to the the books, and while it incorporates material from throughout the series, it's actually telling a brand new story with a new ending not seen in the books. There's also the unorthodox follow up planned for the film. While no sequel to the movie has been announced yet, there is a follow-up television series that has been confirmed. The TV series will not be a sequel, though, it will instead help to fill in some of the backstory of the film, acting almost as a prequel.
Given that the movie ends in a new place compared to the books, and that the next known project in the franchise is a prequel TV show, it makes sense that there is no post-credits scene in The Dark Tower. What would such a scene show? It's not like it can tease what happens in the next movie based on the next book in the series, because that's not how the series is structured, and it's not even clear at this point if there will be another movie, or what the plot of that film would be. It would also be exceptionally strange to preview a prequel TV show that hasn't yet begun production and will feature a mostly new cast, as that would likely just leave everyone in the audience more confused than excited.
So The Dark Tower doesn't have a post-credits scene because it's not a conventional franchise-starter. The film is not trying to be the typical book-based blockbuster, it's doing something entirely different in the genre, and that means letting go of Hollywood traditions like the post-credits scene.