The mmWave functionality requires a glass section on the frame as it can't pass through metals. They've redesigned this on the 17 Pro[1], but likely didn't find a way to integrate into the Air's design.

  When Camera Control was introduced on iPhone 16, Apple moved the 5G mmWave antenna to pass through the back glass of the iPhone, that way it was no longer something you needed to see.

  Now though, with iPhone 17 Pro – that can’t work. The iPhone is now largely made of aluminum, requiring Apple to revert to an old design technique: a glass cutout for 5G mmWave passthrough