Marketing, my friend, is the name of the game. If you could play cross platform, what would the incentive be to buy one particular platform over another? It would be great for the consumer, and I want it as much as you do. As much as they say it is about fair competition (as mouse is far superior to joystick in any game that requires fast precise reaction), it is really about making money. Microsoft wants you to buy their console, and Sony wants you to buy theirs. If they were cross compatible, then there would be no point to buying both.