Because by doing so you’re making an enormous amount of software illegal, and compelling software developers by law to support anything any other developer may want to do. According to your logic, Apps should have full access to ring 0 too.

If public benefit is your only concern, then why don’t you consider the benefit of closed ecosystems, or at least of consumers having the option to choose one? I don’t see any public benefit in removing options for consumers. If open ecosystems offer such a superior benefit to consumers, then surely they would be able to out compete closed ones.