One reason is APFS is designed for SSDs and assumes each disk block has an equal latency to read it.

For those of us with encrypted spinning disks sitting in a fire safe/deposit box for a decade or more, macOS suddenly won't be able to mount them for data retrevial because of a software block. If we want to move our cold storage to APFS encryption, we'll need to replace the spinning discs with expensive flash storage, or live with slow fragmented access.

For comparison, I can still pull out a USB floppy disk or CD drive, connect it to modern macOS, and read a shoebox of floppies/discs inherited from my grandfather.