When you build a house, you start with a dream. Then you hire an architect to create a floor plan and the blueprints and you hire a contractor to hire a bunch of folks from the trades and you watch as your dream is erected.
With writing, you also start with a dream. But then someone blind folds you and sends you to a vacant lot. You have to hand-craft the materials from the very earth you're standing on (or pull it out of certain parts of your anatomy) and you have to guess as to whether you're putting the stuff you've made in the right places. Other people can shout at you and push you here and there, but you really just have to feel your way around. No one else can do it. You don't know what the thing is supposed to look like until after you're done building it, and as soon as you're done, someone will come along and tell you why it will never survive the first storm.
Um. Why do I do this? Someone remind me. Oh, right. It's fun. I forgot.