"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.
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.