1) no one in the camera department can train you or step in and cover you if you were sick etc. There is no apprenticeship model for the DIT in the camera department.
2) the skills required are outside of the departments core skillset
Sorry, didn't reply to this before.....
1. In the US there is a training program, a pretty comprehensive and active one, run by Local 600 for exactly that purpose, at least in part.
2. Many individual skills within the camera department are not part of the core skill set of other members of the department. The skills of a loader or even a 2nd AC have no relationship to focus pulling or operating and don't teach those skills at all. Focus pulling as a skill has almost nothing to do with operating - I know a number of excellent focus pullers who can't operate worth a damn. And I know a number of terrific operators who can't pull focus to save their lives. And none of these jobs prepare you for the ultimate camera department job, Director of Photography, other than to inform you of some of the individual skills involved in pulling off a shot. In many ways, a gaffer is more prepared to assume the responsibilities of a DP than an operator is, despite their coming up through a different department. So I'm not sure I agree that different skills are a justification for a new or different department.