I think the average person today, who knows next to nothing about computers, would rather buy an OEM computer. It's cheaper, has technical support, warranty. They don't really care about the upgradability, and those who do don't really understand it anyway, thinking you can upgrade an OEM computer just as you would any other.
People who don't know much about computers want to buy from a reliable brand. People who DO know about computers would rather build a computer themselves.