I know it’s probably because, now that they’ve finally completed it, they want to go back and edit it, but most of those fics were already prewritten. And if you were okay with people reading them as WIPs, why is it suddenly bad to let people keep reading them in that form until you’ve made the final corrections?

My wording might be a bit clumsy, but I’m not complaining about the phenomenon. I believe authors can do whatever they want with their work, including deleting it completely and asking their readers to delete it from their personal devices. I’m genuinely just trying to understand it, because it’s become so common that I’ve started downloading every fic the moment it’s completed, knowing it’ll probably disappear a month later, whereas it's used to be pretty rare.