Xbox One release date: Q4 2013
PS4 release date :Q4 2013
PC processoers (Xeon E5450, E5440, Q9550, etc) capable of the same projected passmark score as the the current consoles came out Q4 2007 to Q1 2008. This is PC hardware that was released ~6 years previous to these current generation consoles.
Xbox 360 release date: Q4 2005
PS3 release date: Q4 2006
Thinking back to the era of the previous generation consoles (Xbox 360 and PS3) , PC hardware that was six years older (ie, released Q4 1999) would have been an early Pentium III. Unfortunately, it might have been this six year old PC hardware was even slower the 733 Mhz Pentium III found in the original Xbox (released Q4 2001). Therefore using old and cheap PC hardware back in 2005 to 2006 to compete with Xbox 360 or PS3 was out of the question.
So its a unique situation we have today with old desktop processors being comparable in CPU processing power to the newest consoles. Further, encouraging the use of this older hardware IMO is the fact that Microsoft no longer provides support for the original XP licenses found on Core 2 era machines.
P.S. I would even make the argument that a processor such as the Xeon E5440 is a better CPU than the octocore found in PS4 (re: it gets the ~same passmark as the PS4, but with half the cores....meaning single thread on the Xeon is twice as good. The downside, of course, is higher power consumption...but 80 watts TDP for the Xeon is still not bad)