Why does baseball use three strikes and four balls?

Neither number was designed. Early baseball had no called balls at all, since the pitcher's job was to serve hittable pitches and the umpire simply waited. As pitchers began working around hitters, rules were added and then repeatedly adjusted: nine balls for a walk in 1879, eight in 1880, seven in 1882, six in 1884, seven again briefly, five in 1887, and finally four in 1889, where it stuck. Three strikes was more stable but also emerged from convention rather than design, appearing in the earliest codified rules of the 1840s.

What that history shows is that the count is a tuning knob for the balance between offense and defense, and baseball has never stopped turning it. When pitching dominated in 1968, the sport lowered the mound and shrank the strike zone. In recent years it has changed ball construction, limited defensive shifts, added a pitch clock, and enlarged the bases, all for the same reason the walk count moved five times in a decade: the game is continuously refereed toward a watchable equilibrium. The four-ball, three-strike structure survives because it produces a satisfying asymmetry, the hitter needs one good pitch and the pitcher needs to avoid throwing it, which is the essential tension of the sport.