Keyword: Laziness
When you start building a house, you usually build the walls and then the roof, because a floating roof would just be weird. You would then probably lay out the inside so you can see how tall the rooms will be, the room sizes, et cetera. So let's say you are building this house, and you realize that you are one block off, which means you will have to redo the roof. This is really annoying because this takes time you could use to build the windows and whatnot. But you decide it's ok, you can make it work. So then you keep building, and then it turns out everything is wrong and there isn't enough space to put down
anything. So now not only do you have to redo the roof, you have to redo the inside and the walls. Moral of the story, DON'T WAIT TO FIX THE PROBLEM. I have done this so many times, it's unbelievable.
Another way to avoid this is to build out the frame first. Then, if you have any mistakes, you won't have to redo the whole house, just a couple of blocks. Here is a cool frame I built: