Because its not accepted at the ORB or Hwbot and doesnt portray a realistic result when comparing ATI to Nvidia or games without PhysX.
For example, someone comes on here and sees that a GTX260 216 is beating out a HD5850 in vantage b/c physx was enabled, then goes and gets a 260 216 only to be spanked around by games without PhsyX....
Another thing to mention is that it boosts the CPU score, not GPU score.
PhysX when enabled in games without a dedicated physx card, the FPS DROP, while in this benchmark the results are better.
Shall I go on?
(sorry for being so bitter about this)