The simple answer would be because you paid for it and wish to listen to it however you want.
People may just want to listen to them on their chosen player, having no intention of giving it to other people. And when people want to listen to them in the car, they could only burn audio book on CDs. Because of the CD standard, you may need 8 CDs for a 160M audio book file.
People couldn't make it future proof. When Audible.com close down or you format my computer and can't get online to enter your username and password.
If you rip your audio books into MP3 files, all the problems doesn't exist any more.
Run the software and it loads all playable audiobooks from the iTunes Books library automatically. You can also drag aa, aax format to click and select the files you want to convert.
Click the Profile dorp-down list to choose the output format as you need.
To save the converted file, just click the Macsome AudioBook Converter menu and select Preferences to customize the Output Settings by clicking Choose... to set the destination.
Press Convert button or click the menu Control->Start Converting.