It's not just that. E.g. Cooper and Hutchinson's edition of Plato's complete works, available on Kindle for $31.[1] Or on another tack, Yudkowski's recent If Anyone Builds It, Everyone Dies.[2] Neither book is on Unlimited, and I couldn't find either one on Kobo. I struck out half a dozen times in a row and finally gave up.

[1] https://www.amazon.com/gp/product/B00OZ4NMHU

[2] https://www.amazon.com/Anyone-Builds-Everyone-Dies-Superhuma...