Apple does not make their services available on anything other than Apple devices, nor do they allow other providers on their devices.
Regardless of an open format for the content, I cannot choose to use another service without also completely changing platforms, and if I change platforms I cannot use Apple's service.
On the first point again DRM is foisted on Apple against their preference. We’ve seen what an unholy mess multi-vendor DRM systems look like from the debacle that was “Play For Sure” with every vendor blaming all the others for nothing working properly. Can you really blame Apple for not wanting to wade blindly into a quagmire like that?
Secondly, you can absolutely use other services on Apple devices such as playing music bought from Amazon in the Amazon Music app, or using Pandora or Spotify. I use audible on my iPhone, and there's a perfectly usable kindle app. Apple Music works on Android, and you can transfer music bought in iTunes to any other device. Sure there are some limitations, there's no native iCloud client for Linux and it's support on Windows through iTunes is limited, but eh. There's a limit to what I think it's reasonable to expect them to do to make life more tolerable on other people's stuff.
Apple does not make their services available on anything other than Apple devices, nor do they allow other providers on their devices.
Regardless of an open format for the content, I cannot choose to use another service without also completely changing platforms, and if I change platforms I cannot use Apple's service.