This has always been a wonder for me as well. Although after thinking about it from both sides I stand on the side that the defender is just taking up there space. The point of offense is to attack and beat the defence. So you are not going to just sit there in their feet you should swim around them and make a move or a pass to another player. The defender has the right to be there and positioned like that because otherwise then it wouldn't be defence if you would have to move out of the way all the time for the other team.