Otherwise that art-school lesson is rather that every impression in the world can be equally represented by a JPEG on some screen (and while painting is a fun exercise, it's just "a imprecise JPEG with extra steps")...
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.
As examples: Botticelli’s Venus is a much different experience in person, when you see it from across the room. Anslem Kiefer’s paintings don’t hit you nearly as hard via photographs vs. in person. And so on.