1. Most people wouldn't have the desire or expertise to upgrade with third party stuff. Most people don't want to muck about in the guts of their PC. 2. Apple

1. Most people wouldn't have the desire or expertise to upgrade with third party stuff. Most people don't want to muck about in the guts of their PC. 2. Apple could sell those parts themselves at a much more reasonable profit margin than they do today. 3. Better to sell the cheapest option (at a good profit margin) than nothing at all.