Why doesn't a skydiver land directly beneath the point where he or she jumps out of the plane?
The skydiver has an existing velocity relative to the ground when she leaves the plane.
The plane is flying at - well certainly more than 100 km/h and maybe quite a lot more.
When the skydiver leaves the plane, she is moving with that velocity relative to the ground.
Air resistance will slow that horizontal motion so that in the end the motion is mostly vertical, especially once the parachute is open, but in the mean time the skydiver will have traveled some distance in the same direction that the plane was flying when she jumped.