I read somewhere that due to the average distance people sit from their speakers the high frequency tends to be diminished as it travels through the air to the listener. Recording studios may boost the high frequencies to compensate for this. If this is true then of course all headphone listening becomes too bright as the treble energy is not dimished travelling across the tiny distance inside a headphone. It is perhaps strange that darker headphones are not more common as headphone manufacturers are always producing unbalanced designs to give the customer what he wants rather than accurately reproduce what was recorded. Perhaps there is not as much of a market for dark headphones as there is for v-shaped ones or even mid boosted ones. I found my Sennheiser HD595s to be dark sounding.
It's not so much that the treble energy is diminished as it travels, at least not totally nor primarily. The one problem with headphones is they sit smack over your ear canals. Position speakers similarly at home - at the same distance from you, but put them at your flanks, firing directly at your head like how flanks fold over the enemy's center salient. You'll very likely develop the same issues with treble.
Even door speakers in a car aren't mounted that way since they're technically around 2ft or more in front and below from your head, not level with it and firing directly into your ears. Angled driver mounts or earpads can reduce this but if the driver itself already has peaks in the treble (as most of them do, even the darker DD headphones, the peaks are just smaller and broader) then all it does is not make it worse. Try AKG K701s with normal earpads and you'll exacerbate the treble response (or at least, exacerbate the perception of it).