If they don't have a GPU that implements this then the decrypted material would be available to the OS, which is precisely what the streaming media platforms want to avoid.
It's because they don't trust the OS not to leak the decrypted bytes, but they could thanks to TPM (the OS would have to have a memory memory chunk not readable from admin/root etc). In this scenario the OS is part of the DRM.
Mandatory where? Most of the devices people are streaming video to these days aren't even PCs!
Macs haven't had TPMs for a while now (I think Apple never really used it and dropped it even before the Apple Silicon switch), but of course they have their proprietary equivalent.
Yes, but Windows isn't where the vast majority of people watch Netflix, so I don't see the incentive for media DRMs suddenly also making TPMs mandatory.
