Because it's really hard, and really inefficient, and really hard, and the goals of most offenses are to score as close as possible---layups and threes just beyond the arc---and it's really hard. If you chuck a 50-footer, the defense has prevented a layup and allowed a really tough three. Plus, if you do it early in the shot clock, you've prevented the opposing team from even trying on defense. Plus most good defenses pressure the ball when it's brought up the court. This sounds like a question that would be asked by Antoine Walker...