Why doesn't iPhone 12's loudspeaker switch automatically during FaceTime video calls?

During the FaceTime audio conversation, the interlocutor with whom I have a dialogue turns on the FaceTime video, at this time, the sound from the speaker that is attached to the ear becomes very loud and I almost go deaf several times, why doesn't the sound switch to the speakers that are near the charging when switching to a FaceTime video call. Why didn't Apple provide this, if the sound of the FaceTime video call comes from the speaker that is attached to the ear, there should be a quiet warning signal that the volume will increase now. I have an iPhone 12

[Re-Titled by Moderator]