Nobody knows, but my guess: Apple is hiding a test of advanced battery fault detection and monitoring technologies that will eventually show up throughout their battery-powered products. Final Boss Lightning was the simplest, least risky way to support the power delivery and communication needs in the AVP Mark I timeframe.

https://patents.google.com/patent/US8986866B2/en?oq=8986866

They don't want you to use a replacement battery they didn't sell you

If I had to guess, one group came up with (reasonable) requirements for this connector to handle certain currents and voltages, be reliable, be nice-looking, and be proprietary for the purpose of not dealing with RMAs related to people connecting shitty usb-c batteries directly to the headset. And another group realized "hey, we have all kinds of documentation, test results and tooling, we can make a wider lightning cable with low risk and low cost. Sure, we are moving away from lightning but since this is internal to 2 apple products it doesn't matter."

apple doesn’t care.

with this connector, no one will be accidentally plugging the headset straight into a usbc power adapter without the battery in the loop. at least not without some third party help.

Adding latching to USB-C would largely defeat the purpose of using a USB-C connector, because you'd be throwing away most of the compatibility that the USB-C connector could win. At that point, the USB-C connector would be there merely to check a box on the spec sheet, rather than because it's the right connector for this purpose.

As for a battery or supercap in the headset itself: by all accounts the headset without a battery is pushing the limits of what is a reasonable weight. There needs to be less stuff in the headset, not more.