Most pc owners will not install upgrades themselves. If a shop is asked to upgrade then not only does it cost more money but the owners of the pc have the hassle of having to disconnect the pc and take it to the shop for the upgrade (and sometimes having to collect 24 hours later).

If you are a gamer then you are generally clued up about doing upgrades yourself but if you are a the type of person who plays the odd game and uses a pc for office work then just because a graphic card is cheap doesn't mean you are immediately going to install one as soon as a game runs badly. You are more likely to return the game.