That’s because most missed calls are balls and strikes, which will decidedly not be in instant replay’s bailiwick. Brian Costa of the WSJ:
When assessing whether a pitch is a ball or a strike, umpires get the call wrong around 8% of the time, according to a review of every call this season (through Sept. 10) by Inside Edge, a professional scouting service used by 15 major-league teams. That translates to an average of 8.8 incorrect calls per game behind the plate … Excluding balls and strikes, MLB research found that umpires miss only one call every five games, according to Atlanta Braves president John Schuerholz, who helped devise the new replay system. Yet the massive expansion leaves balls and strikes outside the scope of replay.
Yet we’re going to get challenges to a lot of calls anyway simply because giving a challenge system to a manager will demand its use.
I’m prepared to be surprised, but if I were a betting man, I bet games get longer due to expanded replay, not shorter. The system MLB is proposing basically demands it.