There's plenty of reasons to not use a photo or a scan, but some trivial reasons are: translucent, metallic/pearl, and fluorescent paints and pigments.
Other reasons include gamut limitations, interactions with light (as you've noted already), and the point of the exercise is to capture minor details and attempt to understand the artist's original decision making processes. That can involve looking at the artwork under different lighting conditions, from different angles and so on.
Photos of course make good reference pieces, but snapping a photo or downloading a scan of a piece is going to leave many frustrating questions unanswered.
Despite the outcome the goal isn't to reproduce the piece. It's to emulate the process that led to the piece.
Other reasons include gamut limitations, interactions with light (as you've noted already), and the point of the exercise is to capture minor details and attempt to understand the artist's original decision making processes. That can involve looking at the artwork under different lighting conditions, from different angles and so on.
Photos of course make good reference pieces, but snapping a photo or downloading a scan of a piece is going to leave many frustrating questions unanswered.
Despite the outcome the goal isn't to reproduce the piece. It's to emulate the process that led to the piece.