I find it interesting is that every statement like this excludes (or more frequently, omits) the cost ($80-100 or higher) of a legal Windows installation. Most people run Windows, and prefer it to be legal. Then you have to tack on labor - even if you only count active work to build it, it still takes a fair amount of time. Combine that with the illusion of support and warranty, and those $300 PCs (probably closer to the $260 ones) are a more attractive option for most people.
Plus, I've seen a lot of self-built PCs. Biostar boards, Apex (or worse) PSUs, unbranded RAM, and no testing. Almost all would've gotten a better product if they'd just bought something off the shelf- even Acer makes better systems than that. Granted, I've seen DIY systems with ASUS/Gigabyte/etc, but those tend to be even more expensive.
The only market segment where it makes financial sense is the high-end of the market. All major OEMs have razor-thin profit margins on the low-end. They make their real money on the high-end. When you get to the $1000 range, you can build a substantially better machine for a lower price, Windows and all.