Why should the knees cross at ground contact?
A simple way to assess running mechanics is to see where the trail leg is relative to the lead leg when the foot contacts the ground. During acceleration, the trailing knee should align with the opposite foot when it contacts the ground. During top speed, the knees should align when the foot contacts the ground. This alignment is necessary to cycle the legs fast enough so that the foot can be pulled backwards before contacting the ground. If the alignment does not occur, then the trail leg will take longer to rotate forward and may not be able to pull back before ground contact.
The knee alignment facilitates cycling, but this position is also important for complementing the forward movement of the body. The alignment is achieved with active glutes and hip flexors scissoring the legs together while the legs are still in the air, and meeting at ground contact. This results in the lead foot accelerating to equal the ground speed and prevent braking. Both legs accelerating toward each other produces more horizontal force on the ground. Compare it to a single-leg vertical jump using the free knee to drive upward. The inertia of the knee contributes to the height of the jump. Similarly in running, the inertia of the knee contributes to the forward movement of the body. The difference in running is that the inertia of the knee is horizontal as it passes the other knee rather than the vertical movement in a jump. This forward inertia combined simultaneously with the opposite foot contacting and pulling on the ground can be thought of as a horizontal jump. The legs can already be moving fast when the foot contacts the ground under the hips allowing for immediate and maximal horizontal force as well as a short contact time due to the speed of the foot.