Digital codes kind of tried to adress it, but it was too fractured between services to really catch on for the average person. I've given away countless digital codes for that reason. They also expire which is a problem for older releases. In addition, they're not immune to licences being pulled or services being merged/shutdown.

Aside from being an anti piracy measure I think it's also to keep the customers in their ecosystem. Once the customer loads up the app/software to view digital purchases they'll likely be presented with other content to rent/purchase at some point during navigation.

As to why music DRM is less intense, anyone's guess is as good as mine. I suspect it might be that audio files are accessed and moved more often between devices. If the DRM was too hardcore it could become a hindrance and possibly impact sales.

There's a rough road ahead for this kind of stuff I think. Acquisitions and rights being transferred around will result in content being pulled from services, or outright removed from personal digital collections as we've seen a few times already.