Replacing Sports Referees With Technology?

dividedsky319 asks: "There have been numerous instances in which fans of a sporting team blame the loss of a game on the refs. Yet, nowadays, technology could replace a lot of what referees do. A sensor in a baseball could determine a ball or a strike. Same with a tennis match, the ball is either in or out. A sensor in a football could determine whether the ball moved forward 10 yards for a first down. Why hasn't this happened, yet? Obviously not all calls can be determined by technology, but it is feasible for certain instances. What would be the ramifications if something like this WAS introduced, and why has it taken so long?"