bilateral custom foot orthotics are normal why aren't bilateral afos?

Custom foot orthotic or even prefab foot orthotics are typically dispensed as a pair for a number of reasons:

The first as you noted is that providing only one would not doubt create a Limb Length discrepancy.

Second is that many, not all, biomechanical related issues and thus foot symptoms are symmetrical and hence most foot orthotics, whether custom or not are dispensed as a pair.

As for AFO's, these are much more specialized devices and typically the issue(s) which the AFO will address is only present unilaterally.

This include such diagnoses as tendonitis, tendonosis or tendon ruptures, stroke with drop foot, fractures, sprains, usually effects only one side, etc. Thus it is not typical to dispense AFOs bilateral. Even pre-fabricated AFO's when was the last time a patient required bilateral CAM walkers at the same time, unless they had bilateral elective surgery or bilateral trauma? When was the last time a patient required bilateral plantarfascial night braces? Could it happen yes, but it would be rare! And do you think your patients would be compliant?

Second: Is the resolution of an iatrogenic limb length discrepancy:

The limb length discrepancy which is created by the AFO, can easily be addressed by providing the patient with a blank insert or prefabricated foot orthotic of the same thickness as the AFO shell on the contralateral side. This will more than compensate for the thickness of the AFO and create a functional limb length symmetry.