Because it's not that simple? Andreoli is, by all accounts, an MLB hitter (maybe not for long), if he's seen the same pitch 3 or 4 times in a row now he might be able to get the memo and be ready for it on a 5th time, or maybe he could have been guessing slider all the way and a fastball would have blown him away. With a two-pitch pitcher, whose two pitches are both plus, he was probably sitting on one of them, but throw it down the middle and an MLB hitter is not unlikely to hit it.