That's probably because they are practically most used for the underlying OS of a Kubernetes host (seeing as how it is difficult by definition to configure an immutable install).
If you really think about it, what's the difference between spinning up a VM with a preconfigured image and spinning up a VM with an _immutable_ preconfigured image?
The difference is that one is immutable and the other is not. One can be rolled back to earlier version while retaining user data and the other doesn't offer that ability.
Despite the name, that's not what immutable distros are for.
GKE won't let yet restore previous generation of configured and component versioned base image.
If you really think about it, what's the difference between spinning up a VM with a preconfigured image and spinning up a VM with an _immutable_ preconfigured image?