The goal keeper needs flexibility to be able to respond quickly to the ball hit in the goal direction (by jumping in air or going down to ground) so that he/she can catch it and avoid the ball hitting the goal.
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Because of how they often have to move, like diving to reach a ball or suddenly having to change direction when the ball does. The also need to be able to jump very high and catch the ball or punch it. They often need to come out and spread themselves in front of the player trying to score, to make it more difficult to score.