Q: Why doesn't iOS put the phone into Do Not Disturb mode during phone calls?

Does anyone actually want to have text, calendar, or other app notification sounds in their ear in the middle of a phone conversation? Why is that the default behavior?

At most, the phone should enable a subtle call waiting tone, or some indication the battery is getting low. But why would anyone want those other notifications to interrupt their call, let alone the vibration directly on their cartilage?

Was this a popular feature with earlier iPhones so it just stuck all the way through iOS 9?