there a couple goals a game and Hockey already has stoppages. There are 250-300 pitches in a baseball game. stopping to check every possible strike 3 even would lead to ridiculously long games.
They can tell where the ball goes over the plate, but the strike zone height is different for every player. It's knees to the lower chest which means it's different for a guy who is 5'5" than it would be for a guy who is 6'6"