Well, not quite _every_ time. For example, I’m deliberately not doing POSIX with my latest one[0], so I can be a bit more experimental.
At this early stage, the filesystem exists only to prove the disk drivers and the IPC interfaces connecting them. I chose FAT32 for this since there has to be a FAT partition anyway for UEFI.
The concept of the VFS may stick around as a useful thing, but it’s strictly for storage, there is no “everything is a file” ethos. It’s entirely a user space concept - the kernel knows nothing of virtual filesystems, files, or the underlying hardware like disks.