That's because the pitcher is only one person, and can easily be off or on on a particular night. Meanwhile, it is very unlikely that an entire lineup is having a good or bad night, simply because there are 9 of them and the odds even out. This is why the pitcher is 45% of the game while each hitter is 55/9 = 6.11%. This is asuming that they split the 10% that is fielding equally, which of course isn't true. However, overall pitching and hitting are equal, I believe, and in an individual AB, both the hitter and pitcher have equal control over the outcome. You know the 90% is just an expression that's been used throughout MLB history ? I never broke it down literally b/c there's no way to prove or disprove it, I don't think it's near 90% though. If I say "I'm so hungry, I can eat a cow", is someone going to debate the merits of me being able to eat an entire cow? Pitching is more important than hitting b/c pitching b/c being on offense is a defensive position from the get go.