Seen this a couple times. As Sirtel mentioned, it's that pesky Kindle Unlimited exclusivity clause.
Several of Andre Norton's
Witch World collections went on sale recently, and so I grabbed them. Then I decided I wanted to read the original (before starting the various spin-off cycles), but could only find it as an
audiobook on Kobo. A quick search on Amazon showed why:
https://www.amazon.com/Witch-World-S...dp/B078K2D1B4/
Kindle Unlimited strikes again!