It's simply because Audible's business model is based on subscribing to their service and not purchasing individual audiobooks.

I filled out the 'See a missing book on BingeBooks?' form. Thank you for allowing readers to recommend book to be added to the site, an incredible feature! ;)

Generally we're able to add a missing book within a day or two.

You can also just email support@bingebooks.com and we’ll get something added. Thanks!

Hi Bobbie. We're still doing some housekeeping and cleanup ... we have automated feeds from different sources and sometimes they don't talk to each other. Yes, each book should be listed once, with the various formats at different price points.

Sometimes series come across incomplete from publishers, and we have to go in by hand and add titles that are on our site but aren't attached to the series. When you come across one, let us know here and we'll fix it. thanks!