I don't think it's so much about the parts, but about building and upgrading. Not everybody knows how to upgrade and places like Best Buy charge a mint to do it for you. Also, most people don't know anything about Newegg or ZZF and buy stuff from big box retailers.
Most shops are also in the business of selling older equipment, at least around me. So you can pay maybe $200 for a Northwood P4 system, just fine for surfing and office work, rather than $600 for a brand new pc.