Because tons of media that was never properly digitized for the streaming era and only ever ended up on discs.

Doing it now will prevent a loss of history, much like early BBC recordings are lost because they would just tape over old broadcasts to save money.

For example, there was recently unearthed a single episode of a sketch comedy show made by Graham Chapman and Douglas Adams.

Problem was, the tape it was on was from the formats before VHS and Betamax. While the tape existed, no players to play back the tape existed anymore. It took a several year effort to build a new player from scratch. Finally, after all that, they were able to record the show to digital media and now it lives on YouTube for people to see. It's not the funniest material ever produced by either man, but it's definitely a piece of history worth looking at if you've ever enjoyed Monty Python or The Hitchhikers Guide to the Galaxy.

Attempts to digitize things that are currently available on disc but not available in digital file formats/streaming is absolutely a process of maintaining historical documents that would otherwise be lost to time. Building a new DVD or Bluray player from scratch when none exist anymore is a much bigger effort than making a tape video player, because it involves proprietary codecs, compression, and DRM.