There's a nasty tradeoff between having it all look like a single disk and letting the user see the disk structure. Disks fail, or go into pre-fail state, and you want to be able to pull them and replace them. You also want to be able to move them sometimes. In addition, you don't want runaway applications that fill up the disk to stop other critical apps from working, like getting E-mail.

For a giant archive, one big volume may be fine.

There is, of course, another way to do the vinyl job, the way mp3.com did it before they were sued out of existence for doing it. Namely you prove you have the vinyl and you get MP3s or Flacs back. It's not legal, but the studios could do it themselves, and there was even a program for a while where you could bring in your old disks to record stores and get replacements for a discount.

The latter method solves the problem the RIAA worries about of people borrowing a friend's disk and thus getting free music, in that the store takes the vinyl or marks on the label with a pen. But it only would work for stuff that has been digitized already, and only for studios that would participate. And right now they would rather sell you your music again at full price (to fewer people) than let you have it at a discount.