Why are there only 9 innings in most baseball games unless they need to break a tie score?

Major League Baseball, Minor League, and most college games are 9 innings in length, because that's the rule. Some college games will be only 7 innings, but generally only if it's the first game of a double-header. Most High School games are 7 innings. PONY also plays 7 innings, and Little League (12-years-old and under) play only 6 innings. Howrever, until the High School level is reached, most games are played with a time limit established by the local league.