A reason often cited for making your own Kindle cover is cost, but some truly nice covers are under $20. The real reason for doing it is that you love working with your hands, love the feeling of creating something all by yourself. Me too.
My favorite Kindle cover style opens and is held like a book, which leads to making a cover using a real book. The method is quite common, and whoever thought of it first has an ingenuity gene. Here are two links so you can see the basic concept: