I'd say it's because Sony doesn't know how to make games that appeal to people's tastes going by your criteria. Of course you are using a silly definition of exclusivity anyway.
Not appearing on competing platform != exclusive
If you were to compare games made by third parties only on one platform it'd be Wii / DS / PS2 / PC and then everything else miles behind.
People are difficult to govern because they have too much knowledge.
When there are more laws, there are more criminals.
- Lao Tzu