Having to pay to have iWork
Why do I have to pay to have Pages, Keynote and Numbers?
MacBook Pro (Retina, 13-inch, Late 2013), OS X Server
Because you may have the iWork '09 DVD which was purchased separate from the OS X App Store, or you do have an App Store purchased copy of iWork '09 individual applications installed on your Mac, or your Mac was purchased prior to Apple offering Pages, Numbers, and Keynote for free on new hardware purchases.
Have you read the product reviews for Pages, Numbers, and Keynote in the OS X App Store? They simply are not up to Apple's distant past of quality and functionality. They are a set back even at free.