Why is related_name required for annotation?

I'm struggling to understand why related_name is required in order to do annotation.

When trying to obtain the count of related object as annotation, one could use the below:

class ModelA(models.Model):
    (...)

class ModelB(models.Model):
    model_a_fk = models.ForeignKey(ModelA, related_name="model_bs")


ModelA.objects.annotate(num_model_b=Count('model_bs')).order_by('-num_model_b')

Why is the related_name required on the model and can't we just use the below?

ModelA.objects.annotate(num_model_b=Count('model_a_fk_set')).order_by('-num_model_b')

As this yields following error:

FieldError: Cannot resolve keyword 'model_a_fk_set' into field. Choices are: id, (...)

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