The studios don't want you to be able to copy your downloaded movie to a USB stick/plex server/etc and share it with your friends and family. They want everyone to buy their own copies.

Music companies were forced to allow DRM free music downloads because there was a big consumer demand to be able to e.g buy a song from iTunes and play it on a Nokia phone, but now the market has moved to mostly streaming and you can easily stream on any device that demand has gone. People who are concerned about actually owning their movies are a declining niche that the studios can ignore.