The lightning headphones are accepting a digital signal and have a DAC inside the cable. Even with the correct physically-wired adapter (you could create one by bridging a regular USB to Lightning adapter to the Lightning headphones, if you wanted), a computer attached to them would see a USB DAC, and would need a driver for said DAC. Until such a time as such drivers are written, for every OS, that adapter wouldn't function the way people expect it to (i.e. universally), so they don't bother making it.
"Not having a headphone jack" doesn't imply "Lightning connector headphones", though. Plenty of other phone manufacturers are also shipping phones without headphone jacks now. They tend to ship with earbuds that have a micro-USB or USB-C connector; which have a DAC inside; and where said DAC uses the USB audio standard.
That being said, there's nothing stopping Apple from making earbuds with a Lightning connector that are USB Audio compatible as well. There's nothing about the "Lightning protocol" that makes this impossible. They just... didn't, with their current Lightning earbuds. Maybe the number of people it would benefit didn't outweigh the cost-savings of eliminating USB signalling components, or added too much audio-latency, or something.
