Rootkit implies it's trying to hide its presence. DRM software does no such thing. It simply wants to assert greater control over the hardware and restrict the user from executing some action in some way in exchange for access to something you wouldn't be able to have due to lack of trust. In the case of anticheat, many do not find its existence malicious or anti user.

They kinda do hide their presence. You install a game, maybe see a little splash screen with an anticheat logo in a corner. You wouldn't realize that you've just installed something with such great access over your operating system.