I build computers all the time for clients, you get far better value for money building as you spend every penny of your money on branded quality components.
For example I know somebody who just paid, £600 for a compaq with 4gb of ram, an i3 processor not even sandy bridge.
But for £620 you can spec an 3.1ghz i5 with 850mhz built in graphics, 8gb of corsair 1333mhz ram (top end stuff as well) and an 80 plus certified 600 watt psu and SSD hard drive for windows and separate data hard drive.
And with the ssd drive it will boot up considerably faster than a shop pc.
But the compaq you would be lucky to get a 500watt psu and it will be uncertified and from some Chinese manufacturer nobody has heard of.
As they chuck em out for next to nothing and charge you as much as it costs to build an up to date decent spec pc..most people in the know will know that almost all.. Not all but almost all shop mass produced computers are obsolete before you even unpack it. Like processors some manufacturers are still not using sandy bridge professors.. They just advertise it as an i3 i5 etc and to those who don't know you are getting a 2008 processor instead of a 2011.
Big name brands due to mass production are slow to react to new tech, and also because a newer variant pushes down prices of the old they will continue buying the old in order to maximise their profits