Why doesn't IK of arms work if the target object is a child of character bones?

Question

Unity

I try to keep the character's hands on the weapon:

animator.SetLookAtWeight(1, 1, 0.5f, 0);
                    animator.SetLookAtPosition(Application.instance.characterLookAtPoint.position);

                    if (leftHandPos != null && rightHandPos != null)
                    {

                        animator.SetIKPositionWeight(AvatarIKGoal.LeftHand, 1);
                        animator.SetIKRotationWeight(AvatarIKGoal.LeftHand, 1);
                        animator.SetIKPosition(AvatarIKGoal.LeftHand, leftHandPos.position);
                        animator.SetIKRotation(AvatarIKGoal.LeftHand, leftHandPos.rotation);
                        

                        animator.SetIKPositionWeight(AvatarIKGoal.RightHand, 1);
                        animator.SetIKRotationWeight(AvatarIKGoal.RightHand, 1);
                        animator.SetIKPosition(AvatarIKGoal.RightHand, rightHandPos.position);
                        animator.SetIKRotation(AvatarIKGoal.RightHand, rightHandPos.rotation);
                        
                    }

everything would be fine, but if the weapon becomes a child of the skeleton, then the hands stop moving.