Writing, fiction writing especially, requires a good deal more self-discipline than other professions. It's up to you to determine where, when and how much writing you should do in a day. This is different from an office job, or even owning your own business where a great deal more structure is dictated.
Also, writers are creative. Creative people are generally somewhat dreamy, they spend alot of time in their own heads, not really paying attention to the outside world.
Long fiction is a very time intensive form of creation. generally speaking, writing a book takes longer than just about any other of the fine arts. I am aware of the exceptions to this, but most painters, sculptors, ceramicists I know don't get six months to work on a commissioned piece. They're lucky if they get a month.
So to write a novel you need; A great deal of self-discipline from a rather distracted personality type, over a time period much longer than most creative people are ever asked to focus. All of which doesn't even take into consideration all of the craftsmanship required before a person can be considered to turn "writing" into "Art/"
