(This is purely speculative, but…) another reason could be that they simply don’t feel it’s relevant to digitally delivered music.
A booklet is generally considered a standard part of a CD purchase – it includes the front cover to attract buyers, it provides information about individual tracks of the kind that’s needed to ensure radio stations to pay the correct royalties, it might contain an artist biography, or maybe some photos. All of which is either not needed when you use a streaming service, or which there are alternative equivalents for. And if you think about it, does a PDF booklet (which is something intended for print) really make sense when viewed on a screen?
Or is there perhaps a better way to deliver a similar kind of value to the user (for instance, a link to read an artist biography, or a gallery of artist images, both of which I’ve seen on Spotify)
That all said, I do feel streaming services are missing something important in the experience they provide to listeners – there’s something we used to get from picking up an LP or a CD that we don’t quite get from the digital services, but I personally don’t think a PDF booklet is likely to be the best way to fill that gap. I’ve never used Roon, but I suspect this might be something they’re trying to achieve in their own way.