Ignorance of the masses; Very few people realize when they get an HD Voice call that it's because they're on the same network.
It's frankly surprising that people make regular calls anymore because of the inter-network lack of HD voice; you'd think everybody would make hangouts or FaceTime calls.
Of course, the other likelihood from ignorance is that few people have good hearing.
The problem with "everybody would make hangouts or FaceTime calls" is that not everybody has unlimited data. Of course it depends how much you use your phone for calling while off wifi and how much data you pay for, but it's definitely not a factor worth ignoring.
It's frankly surprising that people make regular calls anymore because of the inter-network lack of HD voice; you'd think everybody would make hangouts or FaceTime calls.
Of course, the other likelihood from ignorance is that few people have good hearing.