The reason I can see is to make more money. I have bought a Macbook Pro with more RAM than I needed at the time only to be sure to have the RAM in case I need it later. Otherwise I would have waited and bought the RAM later. I am sure I am not alone.

With closing off everything they have more control and can cut out third party competition. It looks like they are successful with IBM's Micro Channel architecture strategy of the 80s.