Why can't I get Audible books into iTunes?

This Audible help page offers ways to do this, depending on what your device is. It does not mention how to connect Audible on a Windows machine.

https://help.audible.com/s/article/how-to-listen-to-audible-audiobooks-with-itunes?language=en_US

I’m a long time Audible listener and have always found it much easier to listen via the Audible app. If this answer is not helpful, you may want to contact Audible Customer Support directly. There is a phone number.

https://www.audible.com/about/contact#