Why stories never ever go to plan

Does anyone know? I don't. I wrote a plan for the next part of When All Was Fair, and within about ten minutes of starting to write from it, I was having to change it and realising that various characters were not behaving as the plan said they would. Although technically Lecker wasn't mentioned in the plan. Technically. But yeh. Oops. Never mind. Provided it ends up ok, and I actually finish the thing at some point, that's fine. It's over a hundred thousand words already, and I'm thinking I'm maybe half way through. At the moment I've got the MCs in 1929 living with the wrong people, which needs sorting out. And in the 1916 part of the story, I have to stop Dirk and Strom hating each other. And maybe put Ratin and Strom into the same room. And have Erna found out. And stuff like that. So there's a lot left. I've discovered, while writing this, why historical fiction tends to be quite long, especially when it's set in two different parts of time. It's because the nature of the story makes it so. You have to get in some good detail about the period, you have to do research into it, and with this I have technically three sets of characters: the Germans in 1916, the British in 1916, and the Americans in 1929, which kind of gives me an excuse for making it massive. Anyway. If anyone has any theories about why stories never go to plan, please, tell me.