Some people have more money than time. Only takes a few minutes at a shop to buy a book, but reading can take an hour or more for a larger book.
Me, I'm the other way around, though I do have a pretty huge collection. Though I did once find a book in my collection I don't remember buying - one of the Fading Suns d20 books. Once I re-discovered it, I read it, but I have no idea when I bought it, so I don't know how long it sat there.