Headphones are stereo (okay, there are some exceptions), and optical S/PDIF can do stereo PCM, so there's no quality downside. But for home theater audio, you can't do any lossless multi-channel formats over optical S/PDIF, so the vast majority of enthusiasts will use HDMI.

It’s (likely) because they don’t want to add a secondary hdmi output for audio like a lot of other high end gear uses.

Never would have guessed that for headphones! I guess they have some kind of a dock?

Wireless (non-Bluetooth) Sennheiser headphones also take optical input (as well as 3.5mm). Since they're only doing stereo, and require a digital-to-analog conversion on the headphone side, makes sense to avoid an additional analog-to-digital on the transmitter side.