Written manuals require skills often lacking in photographers and programmers, so it probably requires some degree of "outsourcing". In addition, it is difficult to write a manual that is truly useful to the wide range of knowledge and learning styles among the users.
The best manuals I have seen were written in the 1980's for word processors. That was made possible by the high prices ($300 for a DOS wordprocessing program such as XyWrite), and by the fact that word processors were working with a well understood set of tasks. While Adobe has maintained the high prices (and hence resources to spend on documentation), it has allowed Photoshop to balloon into such a monster of functions and arcane terms that it is often difficult to guess how to find the information you need. It is a pity that it drifted so far from the processes and language developed over many years by film/darkroom workers.
A well thought out set of help screens is often all I need, but even Adobe, with all of its resources, does a terrible job at this, largely because of unintuitive language and functional complexity.