Own audiobooks not syncing to iCloud from Apple Books

Why doesn’t my own audiobooks sync to the iCloud, but my own epubs do?

when I import an audiobook into macOS books Apple it does not sync to the iCloud services, so I am unable to listen to the audiobook on my phone. This is annoying as I have loads of audiobooks I have bought from before Apple started doing audiobooks but I cannot listen to them as easily.

The only way I can listen to audiobooks on my iPhone is by plugging the phone into my Mac and syncing it via the file app, I cannot put my own audiobooks on my phone any other way.

also because of this when I finish the book and remove it from the storage to listen to another audiobook, my reading progress then disappears from the total score at the bottom as the audiobook is no longer on the phone.

i do not have this issue with epubs or pdfs as when I open them on my phone or mac it is then available on all my devices.

This is getting annoying as I want easy access to my audiobooks.