This may sound silly, but this makes a real difference. Until this, it was not possible to run standard containers like Docker or Podman in an initramfs without ugly tricks (like using chroot instead of pivot_root, or mounting the root over itself before starting init).
This makes the initramfs rootfs un-special. Aside from being volatile, but this is a design decision.
The truth is hidden in a comment: The goal was to learn fanotify syscall using a real world use-case. This said, when Dockerizing an application from scratch, using an optimized base image may be a suitable option. But that's not always the case. For instance, I often start a project from the Python base image which contains loads of generic libraries that I will not use in a given project but will be important for others. This is when a profiling based approach is interesting. You get the ease of a known environment and the efficiency of an optimized image.
This makes the initramfs rootfs un-special. Aside from being volatile, but this is a design decision.