I find that there's still one aspect to this missing, which is durability and maintenance. I have a suspicion that one of the reasons you primarily see this on higher end headphones is because there's expectations that if you put down 200$< on a pair of headphones they need to be able to last. (Yeah, the HD58X are technically cheaper, but their model design originates in more expensive headphones.)
As Michael mentioned, both cups on a headphone need power. The actual component that produces the sound is the driver, the small mechanical device that pushes the air in rapid succession, creating sound waves. One of those are housed inside each ear cup, and, if it's a normal wired headphones, those are usually the only components that require power. Chances are, that if a headphone has only one cable going into one ear cup, the other one is actually connected to the first one by a cable routed through the headband. This is visible on some headphones, like the HyperX Cloud 1 and 2 headsets. I have not yet seen a headphone made in this way where that cable was easily detachable, which means that if it breaks for some reason, you'll basically be stuck with a headphone that can only play sound in one side unless you have some moderate tinkering experience and spare time to kill. Having each cup receive power through the external, usually detachable wire just means that there's a few internal components less that could fail and cause the entire headphone to need replacement. Buying a 5-20$ cable replacement is definitely preferable in such a case to putting down another hundreds of dollars on a new headphone.
Furthermore, as AudioJR mentions, if you want to upgrade the cable to a balanced XLR connection (I hear this has some bearing on sound quality, if you have the setup to make use of it) I guess it kind of won't work optimally if both drivers aren't fed signals directly through the external cable.
Lastly, the HD58x and its HD600 series siblings are designed and constructed to be easily dismantled, which a wire running through the headband would complicate needlessly. You can basically pull them apart and reassemble them almost without any tools at your disposal. Sennheiser seems to be selling spare parts for their headphones, although most are sparsely available. Still, it's a nice, quality of life feature for something like this to have, I find. Also certainly makes it easier for people who like to paint their headphones to do so.