IMO, headphones are more complicated to design/produce/manufacture.
We humans have had a long history of fiddling around with circuits/electronics, and I think that amps are just "easier" to build DIY compared to headphones. No "art" needs to go into an amp, just solid electronics design. There are way more factors into building/designing headphones (looks, comfort, quality of build, quality of sound, etc.)
It's easier to mod an amp than to mod headphones...there are just too many factors in building phones'.
I mean, i can build a CMoy or Millet like in a couple of hours. Not so much an AD700 or something..