Building a computer system is not rocket science, i'll be the first to admit, however i would not catagorically state its "easy" either. I only started building my own systems after i became professionally certified in computer hardware and viewed such as a "graduation exercise."
About ten years ago I got the A+ cert, the N+ cert, and started on the MCSE before chucking that and going to the local CC to get an AAS in database with a smattering of other IT stuff like a Java course, SQL, XML, intro Linux, etc. Never worked an IT job though. Divorce hit, I needed a job that week, and real estate sales beckoned. But I could confidently build the mechanicals of a system, and get the HD-O/S up and running. The part I have no confidence in is the primary BIOS stuff, any driver stuff deeper than a load disk, and configuring modern RAM. I could learn, but right now I just don't want to.
If one has never done it before, doesn't have all the knowledge and/or doesn't have someone readilly available to guide one through their first build....then it is safer and better to have it built for you. I still recommend steering clear of big name brand machines. Order a custom rig through a small business you trust, or a computer guru you know.
That last is what folks always say, but how many people out of their 20s still know "some guy" who builds computers in his spare bedroom? Years ago I knew guys like that--they had "BYTE" in ink on their foreheads where they fell asleep while reading. (If you don't know what I'm talking about you are very young.) There are no real "small shops" either, at least around me (Maybe SoCal is different.) They either get big and become a DigitalStorm or a CyberPower (both garage start-ups) or they get eaten by the margin monster as they can't get volume discounts, they can't afford marketing to drive demand to their door (and wouldn't know how to do that if they had money--they're tech guys), and the landlord and their banker don't care.
The CyberPowers of the industry fill that niche that "some guy" filled in the late 80s and 90s. Their challenge is to get big enough fast enough to cashflow out while simultaneously building the soft side of the business enough so customers trust them with a $2000-$6000 decision. I think CP and DS can each build a good computer. My challenge right now, if I go with one of them, is to judge which one can do it more nearly perfectly every time, first time. And which one, if they don't, has the infrastructure to correct their errors.