Audio Books Follow-up

Why do publisher buy audio rights if they find it too expensive to actually have one produced? Wouldn't it be better to not buy the rights and leave it to someone who actually wants to create it, so they can also drive more people to buy the paper version?

At the contract stage, the publisher might have a decent idea of how much they're going to put into the book (money and time-wise) and how it's going to do, but also they're secretly hoping they're wrong, and the book might become wildly successful - in which case, they're going to want those seemingly-irrelevant rights because they'll be worth a lot of money. This is why it's the publisher's job to hold on to as many rights as possible, and the agent's job to argue the same on your behalf.