Because the shim is still secure. > Shim then becomes the root of trust for all the other distro-provided UEFI programs. It embeds a further distro-specific CA

Because the shim is still secure. > Shim then becomes the root of trust for all the other distro-provided UEFI programs. It embeds a further distro-specific CA key that is itself used for signing further programs (e.g. Linux, GRUB, fwupdate). This allows for a clean delegation of trust - the distros are then responsible for signing the rest of their packages. Shim itself should ideally not need to be updated very often, reducing the workload on the central auditing and CA teams.