That's the wrong tennis analogy. Women can easily hit the ball over the tennis net. Even children can easily hit the ball over the net. If women were so short that when they served they made contact with the ball at a lower height than the net, you could argue that men and women ought to have nets of different heights. The point is that skilful men, women and children can play tennis on the same court, as it was meant to be played. The same cannot be said of hurdles (it would be ridiculous for women to hurdle 3ft 6 in) and steeplechase (probably to a lesser extent). It's a similar reason why boys do different distance hurdles through the age groups. In British Athletics I believe the hurdle distances are 75m (under 13), 80m (under 15), 100m (under 17), 110m and 3f 3in (under 20).
The serious tennis question is why do men and women play a different number of sets in many competitions?
Similarly there are serious questions in athletics.
Why don't women run full length cross country?
Why don't women do 20km and 50km walks?
It's arguable that women should throw the same implements, particularly javelin.