Because in tennis no-one can predict this. When Connors was 25 Ashe said Connors would not have a long career because of his style of play! No-one predicted he would play into his 40s. No-one predicted Agassi would get to a US Open final when he was 35 when Agassi was in his 20s. I thought Lendl would go on longer than he did. I thought the same of Becker. I thought McEnroe, Borg, Edberg, Becker and Wilander would be slam winners until they were thirty. People keep saying Federer will be like Connors and Agassi and keep playing on. Who knows? With tennis there are surprises about the length of players' careers at the top. You can't make predictions. You have to go by a year by year basis. Federer could retire this year. He could keep going until he's 40 and win several slams in his 30s.