That's just it... Pitcher BABIP doesn't fluctuate that much. We're talking about a .040 gap between the elite and the worst in MLB. As opposed to hitters, where we can see a .130 point gap between pretty-good-to-great players, much less the best and the worst. If you are really bad at throwing a baseball, your BABIP will be higher... But, on the other hand, you'll also let more baseballs leave the park, which do not count against BABIP (hence the .271 BABIP, 7.6 WAR Barry Bonds season I pointed out). Pitcher BABIP doesn't fluctuate at nearly the rate we see for hitters and the vast majority of MLB pitchers are within .015-.020 points of each other.