I work in IT and have all the knowledge to build my own, but instead have purchased my last two machines (both have lasted me the past 5+ years combined). The issue with building your own is keeping the costs down to stay competitive with the OEMs. I found that I couldn't, on paper, get a top of the line (for the time) processor with everything else I needed for much less than just buying the best desktop Best Buy had on the shelf. If I didn't want top of the line, ie- midrange i5 or AMD, then building it myself would have saved me enough to make it worth my time. If you decide to upgrade any internal components, like a power supply, you can toss the spares in a box as backups for after the warranty expires or resell them if you don't foresee using the warranty. I never needed to or intended to, so I resold things like the hard drive, power supply, and memory that came with the OEM computer, which I had better components already. It was all a wash when I figured it out financially later. The OEMs buy and sell in huge quantities, it's tough to compete with that. My processor alone was $350+ new at the time were I to buy it retail.
Consider also the cost for Windows if you don't own a copy you can install on the new computer or migrate your existing install to the new PC yourself. It was still cheaper for me to buy top of the line off the shelf despite having my own copies of every Windows OS out there available to me (legally).