Comment by phantasmish

Playing them, handling them, and poor storage all degrade them. Most original prints will have been played many times, and often haven’t been consistently stored well.

The 4k77 et c. fan scans of the original Star Wars trilogy, which aimed to get as close as possible to what one would have seen in a theater the year of release, used multiple prints to fill in e.g. bad frames, used references like (I think) magazine prints of stills and well-preserved fragments or individual frames to fix the (always faded, sometimes badly) color grading and contrast and such, and had to extensively hand-correct things like scratches, with some reels or parts of reels requiring a lot more of that kind of work than others. Even Jedi required a lot of that sort of work, and those reels would have been only something like 30-35 years old when they started working on them.