Actually having facsimile images of the Public Domain editions online is incredibly useful, and is, I believe, the most platform neutral (despite the presence of lilypond). There is quite a handful of competing file formats for sheet music notation at this point in time. Gutenberg itself accepts a number of them, including at least two proprietary ones (in semi-contrast to their
.txt policy for literary works). Not only is the "population that's capable [of creating computer notated music] markedly smaller than the population who can do this with books", but there are additional problems both theoretical and practical that make this more difficult for even that population, as opposed to the philological problems for text that are generally easy to sidestep in the context of project Gutenberg.