In Porsche's point of view, they want they name out there. EA realeases a game with new content every year with their latest model.
If PD had the exclusive license, we would have a 1960 Porsche 901. A 2002 Boxster and a 2008 997.
We would be wondering were the heck are the great cars or the latest cars. Like we do with Ferrari at the moment.
Where is the Ferrari 330SP? The F50? F355 Challenge? F12? Laferrari? F430 Spyder? 250 California? 575 GTC? 575LM?
The thing we would be doing with Porsche, We would get half a hand of cars and wonder were all the ones we want are.
At least, EA gives people the cars.