Why can't I see iBooks on iCloud.com?

iBooks are supposed to be backed up to iCloud, except when I go to iCloud.com there is no way to see which books & PDFs are backed up there. There are icons for other files such as Keynote, Pages, Photos, etc., but none for iBooks.

I recently lost 20GB of valuable iBooks data (college notes, textbooks, PDFs, etc.) with a recent iOS update (September-October 2023), so have been digging into the weeds on this lately... Apple support tried for 3 weeks but unfortunately still couldn't recover any of the data.

iCloud seems to be the best way to back up iBooks. 1. Creating a backup of your device (iPad, iPhone, etc.) with your mac does not back up iBooks or any of its PDFs. 2. And then "syncing" iBooks only goes from mac to iPad, not iPad to mac. 3. iBooks can be backed up individually with AirDrop, but this is subject to human error, and I often forget to do this if I am too busy or still adding notes to PDFs with Apple Pencil.

So since iCloud is the best option, I'm asking shouldn't there be an iBook icon on iCloud.com so I can verify what data is backed up there?

Apple discussion forums complain about vanishing iBook data, all the way back to 2017. So sadly, maybe the only long-term solution is to not keep PDFs on your iPad.