Primarily because it's a far, far more complicated job than that and you can't really hire someone off the street to do it effectively. Typically in a tech company a tech writer is going to know almost as much or more (after years of experience diving into every detail) about a given technology or application or API, and so that begs the question why not make twice as much working as a software developer and not have to sort out these types of messes?
Also job security. Anyone doing this work full-time is the first on the chopping block, and developers who are working on documentation tend to be perceived as lower status since they aren't delivering features.