Why is that i have to buy the same books for my kindle app on my phone when i already have them on my kindle reader
On the Amazon website, go to 'Your Account' > 'Content and Devices.' In 'Devices,' verify that the Kindle E-reader and the phone with the Kindle Reading app are both registered to the same Amazon account. If you don't see one, or the other, they are registered to different Amazon accounts. Once the Kindle app is registered to the same Amazon account as the Kindle E-reader, select the desired title(s) in 'Content' and 'Deliver' it/them to the phone app using 'Actions' (...). (If the book isn't formatted to be compatible with Kindle app, the phone name will be grayed out and the book can't be read on that device.)
ETA: books you obtained on Amazon are stored on your account and downloaded from C&D. Books are not made available to your device(s) by going to the book's product page if that's what you're doing.