> The disk is fully encrypted, and applications should be isolated from one another.

For most apps on non-mobile devices, there isn't filesystem isolation between apps. Disk/device-level encryption solves for a totally different threat model; Apple/Microsoft/Google all ship encrypted storage for secrets (Keychain, Credential Manager, etc), because restricting key material access within the OS has merit.

> I'm sure these bad ideas come from the busy work invented in corporate "security" circles, which invent complexity to keep people employed without any regard for an actual threat model.

Basically everything in PGP/GPG predates the existence of "corporate security circles".