When it comes to something like (non-adaptation) movies that are very much made with the original intent of only having that one movie, sequels usually range from "eh" to "what the actual hell is this". It's because the world, plot and characters were likely designed only with one story in mind, so adding to that will either have to change or add a lot (different setting, new characters, new mechanics), or require the original to have enough loose ends to go from. Otherwise it'll likely just feel tacked on or a cheaper, worse copy of the original. And if the writers do have ideas for sequels from the get-go, they can't count on them being made, as the first movie needs to be successful enough to get enough want for more - movies far more expensive to make than books, you know.

However, when it comes to stories, you can write as many sequels as you want and if they don't take off, really all you'll lose is time (which is money, sure, but you won't end up shoulders deep in debt for set pieces and actors and advertising and whatnot). Literature also tends to flesh out the characters and the world way more, as it's not bound to a 2-hour time frame or the limitations of only audio and video to tell your story. This makes them stand more on their own rather than in film, where they're more likely to lean on archetypes in order to establish the characters faster so that the plot can happen. When characters (or a setting, for that matter) are given more attention and more dimension, the audience will be willing to see them in more scenarios as they're curious how that'll play out, or they just want to see more of their fave.

On a more convenience-based note, sequels are good for the author as they don't have to come up with an entirely new cast and setting. As someone who has trouble coming up with stuff out of nothing but gets lots of ideas from speculation and extrapolation of existing stuff (started off from fanon and have been building on top of it and my older stories since), that's a lifesaver.