Because the stock cable is a 2.55mm cable or something like that and at one time there was no cable upgrades available.
Then you still have the internal wires going to the driver.
I rewired my HD-598 with Mogami straight to the drivers so that I got 100% of the benefit of a recable.
It's kind of a hassle though and not suggested.
Only cost me maybe $5 in parts though.