Also to note is that Kipchoge is said to be nowhere near being able to touch his toes during a basic hamstring stretch. Sufficient flexibility is important but excessive flexibility can be a disadvantage in running. Or simply a waste of effort.
Finally I'd note that a body's unique biomechanics or exercise history can prevent an individual from achieving the ideal running form. Probably in looking at pros running, you've noticed quite a large variety of forms. If you've spent a lifetime developing one way, especially if it suits your particular body, then no amount of attempting to change it will result in a superior result, even if your body theoretically has a more efficient way to operate. Of course that's not to say that no one has ever made a beneficial correction, but rather just to say that there practical limits.