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.
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")...
Pablo Picasso died more than fifty years ago at the age on 91. Four of his children are dead and the one remaining alive is 76. It's really stupid that people have to go all of this trouble for work done so long ago. Intellectual property laws should not protect your work for the benefit of your middle-aged grandchildren.
A difficult to notice watermark that prevents it being sold fraudulently as real should be more than enough.
It seems like the the people making the film were happy to work with the foundation, for whom this was their first such request.
Even if they had an adversarial posture towards the foundation, the alternative would probably involve a court of law sorting this out, with all the costs of time and money that entails. Even if they were certain they were in their rights to just add a watermark, any risk that could potentially render the film unreleasable for an any amount of time would be unacceptable.
US constitution says copyright duration needs to be finite, but it kept being extended in little increments. Nobody has succeeded in pushing for more than 70 years yet so slowly a few works have been entering the public domain every year including just recently Steamboat Willie.
Macaulay is eerily prescient on the matter.
https://www.thepublicdomain.org/2014/07/24/macaulay-on-copyr...
> even if I believed in a natural right of property, independent of utility and anterior to legislation, I should still deny that this right could survive the original proprietor
> even those who hold that there is a natural right of property must admit that rules prescribing the manner in which the effects of deceased persons shall be distributed are purely arbitrary, and originate altogether in the will of the legislature
> It is good that authors should be remunerated; and the least exceptionable way of remunerating them is by a monopoly. Yet monopoly is an evil. For the sake of the good we must submit to the evil; but the evil ought not to last a day longer than is necessary for the purpose of securing the good.
> the evil effects of the monopoly are proportioned to the length of its duration. But the good effects for the sake of which we bear with the evil effects are by no means proportioned to the length of its duration.
> We all know how faintly we are affected by the prospect of very distant advantages, even when they are advantages which we may reasonably hope that we shall ourselves enjoy. But an advantage that is to be enjoyed more than half a century after we are dead, by somebody, we know not by whom, perhaps by somebody unborn, by somebody utterly unconnected with us, is really no motive at all to action.
> At present the holder of copyright has the public feeling on his side. [...] Pass this law: and that feeling is at an end.
> On which side indeed should the public sympathy be when the question is whether some book as popular as “Robinson Crusoe” or the “Pilgrim’s Progress” shall be in every cottage, or whether it shall be confined to the libraries of the rich for the advantage of the great-grandson of a bookseller