Given that we’re mostly amateurs on these sites, it’s understandable if stories aren’t finished. Sometimes, the author realizes it won’t work out, or, they lose interest due to a new book getting released (Lots of Harry Potter fics were like that when I started into fanfic), RL death/disease/stress (understandable), or many other reasons. I do want to finish my potter fic, but the draw/lure of my own stories is stronger – and my potter fic’s been stuck on a plot point for ages (how to portray something w/o being considered deus ex). In my originals, the main culprits have been plot bunnies! I’ve got WIPs because I decided to split the original story (it held too many competing plot threads, and was better to tell several separate stories than to try to stuff it all under one title). So, I’ve got Fiends (the most direct lineage from the first story), Alaska Trekkers (backstory, complete), Dolbourne Chronicles (backstory, WIP), Jefferey (backstory to a character appearing in the backstory, WIP), and Dale’s Game (backstory that was sparked by a holiday fic, WIP). And, I’ve got some sequels to fiends planned.
Of course, once a story hits the printing press, it had better be complete!
Even if I haven’t truly written the last sentence, I do like to have a good idea early on what that scene will be like, or the general conditions. I try to worry about start/end to each chapter as I write, so that’s a good way to practice, IMO. Frequently, I’ll stumble across the last sentence as I write, not always what I had in mind, but my typing will reach it and it becomes, “damn, that’s a good one – cut!”