Because it's not about the money or having the latest device for me. It's about using what I like - and keeping what I like. I put my old devices to work with new uses and I still get to continue to use and enjoy them while using a later phone as my primary phone.And why not? I've sold every phone from the 3g, 3GS, IP4, IP4S, iP5, and the IP5S. All for $200-$250 more than the phones cost on the upgrade plan ($199-$299). If you truly believe that Apple will sell one phone less during the year if you only upgrade every year and this will save the planet, I can understand that. I don't and as long as the after market will pay these prices for a one year old phone, I'm on board with it.
And if someone I know needs a phone, I have one to give.
I get something back from using the device that is more valuable to me.