The human auditory input channel is the modality already in use by pilots for other parts of this task - for example, copilots will verbally provide confirmation that the plane is capable of landing (flaps, gear). Aircraft safety engineering has done a lot of investigation into how humans process information and make decisions and they have found that putting two mandatory pieces of information for landing (is the ground ready for me, am I ready for the ground) in two separate modalities will inevitably mean that during high intensity moments such as landing a plane, only one modality is checked. You would absolutely get planes landing without clearance, or with clearance but without their gear down.