Because I'm cheap? No.
It's because there is a time limit. If I have the book forever, I don't have the urgency to read it. If I have the book for a limited time I'm under the gun to read it.
Especially with a book on MP3 - it's like I actually have that author with me, but only for a little time. Think about it: If you have Zig Ziglar or Brian Tracy in your car, and only for a limited time, are you going to say "Hey Zig, can you hush for a while? I want to listen to Meaghan Trainor."
Okay, bad example. I would rather listen to almost anything than Meaghan. I'm not all about that bass. But in my case, I will even skip Aerosmith to listen to Zig's advice - every time - if he's on loan.
And of course it's free.