Primarily, it's because I find the exercise a waste of precious time and effort. I still have a mountain of books waiting to be read, and there are so many other titles equally worthwhile that I've yet to discover, so why would I spend all that time reading a book that I've already read?
Plus, I don't enjoy reading a book the plot of which I already know. Every flip of a page, and you have a pretty good idea of what's going to happen next, you might as well recite the dialogue out loud. It's just not as exciting and interesting as when you read the book for the first time. Take, for example, Kazuo Ishiguro's Never Let Me Go. I loved that book so much, but I would never be convinced to reread it even if people say the perspective is wholly different now that I already know how it ends. It's just simply not the same.
What are your thoughts on this? Care to share? :)