I simply don't care how the price of an ebook compares to the price of the same title in a format I don't want to buy. New or old; $7.99 is squarely in my comfort zone for an ebook purchase (especially one I'm almost guaranteed to enjoy -- being by a favorite author).
I can't order a $3.99 used paperback online and begin reading it 30 seconds later.The ability to be able to do just that is quite valuable to me. *shrug*
So I guess the answer to, "Why is backlist (ebook) so expensive?" is ... "me." And others like me.
Sorry.
OK, not really. But I promise I'm quite frugal wihen it comes to food and clothing.