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?
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?