Innings

The Yankees are one of those teams that use formulas for figuring out how many innings to let a young pitcher throw; I believe one thing I've heard is each year you extend a pitcher by 30 innings. Why is this - wouldn't you base it on actual pitches, not innings? Each repetition of throwing is what wears an arm down, not walking from the mound to the dugout, right? And all innings are not created equal. Theoretically, an inning could have as few as three pitches. Pitches per inning fluctuate between about 10 and 30, usually. Say a pitcher throws 120 innings - that could mean between 1000 and 3000 pitches. Which, if you're so worried about babying somebody's arm, is a HUGE difference. Baffles me.