From Offspeed:
The batter is using an implement uniquely unsuitable to accomplish his task. A baseball bat is normally somewhat less than a yard long; it weighs somewhere from 29 to 36 ounces. At its thickest part it is 2.25 inches in diameter. In order to strike the ball solidly, the ball must hit near the center of the bat’s circumference about six inches from the bat’s end. The spot varies from bat-to-bat depending on the type and hardness of the wood, shape and weight of the bat, but at its largest this spot is about five square inches in area.
Think of that for a moment. A hitter must swing a yard-long piece of round wood in such a way that he contacts a small round ball moving faster than he is legally allowed to drive his car. The contact has to occur within a five-square inch area of the wood. The plane of the strike zone varies from hitter-to-hitter but is theoretically 17 inches wide and approximately two feet tall. Of course, the zone is not a plane at all, but a volume of approximately 4.5 cubic feet. It extends from the front of home plate to the rear and a ball passing through it at any point is supposed to be a strike. In real life, the zone tends to be wider and shorter than the rulebook stipulates. Nonetheless, the batter is defending more than four cubic feet of space with a five-square-inch weapon and he has to swing the bat at a speed of 70 miles per hour in order to move it from his shoulder to the center of the plate. “It is far more likely that the pitcher will accidentally throw the ball in the way of the hitter's bat than it is for the hitter to time the pitch perfectly and execute flawless swing mechanics to achieve 100% on-time contact on their own,” according to Perry Husband, who has studied pitcher-batter interactions extensively.