I'm occasionally asked why I charge hourly for most jobs. Some clients would very much prefer a flat fee or to pay by the word, both of which are common payment models for writers and editors. The short explanation is that each project is unique and requires its own special touch. At the end of the day, an hourly charge ensures I get paid fairly for the work I've done to provide what the client has requested.
It's a simple truth that we aren't all equally good at putting words on the page. Writing and editing are highly skilled tasks that require time and effort to acquire and maintain.