The hardest part of writing a book isn't getting started. Resistance doesn't push back on new ideas or even the first few chapters. No, the most difficult part of any writing project is the end.
I like writing books because I like starting projects. It's fun to tackle a new idea. The novelty and excitement of beginning with a clean slate is thrilling.
Mostly, I prefer writing books because they're not ongoing. As hard as it is to complete tasks, it's easier to work on projects with a defined end date rather than something that goes on indefinitely.