It is difficult to tell a coherent story that ends in a satisfactory manner within a preset number of pages. Endings, outside of "and this is how everyone died"

It is difficult to tell a coherent story that ends in a satisfactory manner within a preset number of pages. Endings, outside of "and this is how everyone died", are inherently arbitrary and artificial, and it's tricky to disguise that. And if you're blessed with imagination, there is always one more character to do justice to, one more neighbourhood to explore. (Or, if you're Tolkien, one more etymology to plant.) At the same time, there is a certain type of curious, intelligent, often young reader that delights in those details. They want to know more about that character who spoke two sentences but made a big impression; they want to know how the neighbourhood the protagonist walked through came to be so peculiar. The market for massive 18-book series is a product of the happy match of that type of writer with that type of reader. But it's totally OK to not be that type of reader.