A rider (like a horse) needs time to develop. When riders are asked (same as horses) to assume a position that is more advanced then they are ready for they compensate in undesirable ways. A rider who hasn’t developed enough yet to achieve a long relaxed leg will be reaching for their stirrups which will tip them forward and out of balance. This can lead to habits of gripping with the legs, using the reins for balance, or various other undesirable habits because the rider feels insecure and (of course) out of balance which you now have a vicious cycle.

Balance is the priority and the rider will not progress unless they are in a position that is correct for them AT THAT TIME. Ergo, you need to begin with a stirrup length that allows the rider to find the stability …