Because it's a metric based on a formula created by humans who weight various stats the way they feel they should be weighted.
They are not perfect and admit it.
It's not about being perfect.
While BA is a perfect stat for gauging a hitter's batting skill, it too is flawed due to game scorer's opinions, strength of opposing pitcher, defense and park size.
WAR attempts to simplify player evaluation into one number, so we don't have to say, yes, but player B has more power and is a better defender, followed by, "But, player A runs the bases better and hits better in the clutch", and so on...
It's a flawed number, no doubt, but it tried to do what we all do when arguing who is the better OVERALL PLAYER.