Gregory's Server
They aren't paying for the audio port license and chip. (Yeah, it requires a special chip to decode the audio.) Turns out a Bluetooth chip is cheaper.
ETA: I misspoke. The chip is to authenticate the adapter. But the power lines are always up even if you don't authenticate.
Related: See European Commission proposed laws to require all electronic devices to support USB-C – this anti-competitive move by Apple is one of the reasons.