Other iDevices - like the iPad Pro and higher-end iPhones are being sold for content creation and/or as laptop alternatives. They need 4 lanes of data for connecting 4/5k displays, Thunderbolt/USB4 devices, and even USB 3 only has limited support via Lightning, and have higher power requirements. Macs have gone almost all USB-C (as have the Magic peripherals, although why they ever had Lightning is a mystery).
So although Lightning may be good enough for keeping an iPhone Air charged it makes more sense to adopt the same connector across the entire range.