Few reasons.
AMD chips have been pretty stable in price over the past couple years. I got my 8320 for I believe 160 in September of 2013. They run about 140 now.
GPU's are so quickly replaced that a lot of times you'll see the previous line get dropped in price a bunch when the new ones launch. Then once the supply dwindles and people still want the older cards for whatever reason the price will start to climb again.
PSU, cases, and hard drives are pretty stable in price over the space of a couple years.
SSD prices have dropped a lot.
RAM prices have been everywhere and cost just as much now, if not more, than they did when I first built this machine (first iteration of it) in Sept 2011. I got 4GB for 30-35 bucks IIRC and it's still about the same now.
Motherboard prices also have been pretty stable, although probably declining slightly.
TL;DR AMD product progression sucks and their prices haven't changed much since they haven't released anything new in a while.