Short answer: because different formats have complementary value propositions.
I bought The Loved One, by Evelyn Waugh, twice by mistake. But some other books I buy twice on purpose: I buy the audiobook first and then the Kindle edition (or, if that's not available, the dead-tree edition).
Audiobooks turn dead time into learning (or entertainment) opportunities. There are many cases of dead time when reading from a book is not possible but listening to an audiobook is (repetitive exercise, shopping, commuting, cleaning, and cooking). In the past I listened to podcasts (and still do on occasion), but I find books have a better content density and higher quality.
Fair enough, audiobooks have some advantages, especially for boring exercises like elliptical running. But why buy the same book on Kindle, then?
Because, for the books I have in both formats, the audiobook creates a foundation of understanding and the written book serves as a easily-searchable reference, especially with highlights. For example, I have both formats for Duncan Watts's Everything Is Obvious. I heard the audiobook (read by the author in a very soothing Australian accent) while exercising; then I reread the Kindle version much faster that I would usually read, highlighting the important parts and -- even more important -- getting the references for the underlying research.
So, audiobooks make the Kindle reading much faster and the Kindle makes collecting the essential ideas and the references from the audiobook possible.