Because as I indicated in my 3-27-2017 post possible pathways by which RFI may find its way from the cable to circuit points that are downstream of the ethernet interface include radiation into power wiring, or into other cables, or directly into various circuit points within the DAC or other components. And the degree to which that may occur may be affected by the bandwidth, shielding, and other characteristics of the cable being used. And perhaps also because noise generated by the ethernet interface circuitry at the receiving end may change as a result of having nothing connected to it.
Then I would suggest as T.I. does in "Reducing Radiated Emissions in 10/100 Ethernet LAN Applications" one uses a balanced power supply for the benefits of CMNR.
Bottom line is that selecting well engineered componentry is what is needed for fidelity.
I'm already able to produce ADC'd tracks, that given the natural losses of generational copy, are extremely close to the source PCM when compared. So that means, by fact, my $250 is indeed very transparent and high resolution, and that an $18 NIC, a $69 mainboard, $24 stick of 4GB DDR3, $55 240GB SSD, $25 PICO PSU, $60 LPS is impervious to changes in cable vs some that claim their $8000 device is capable of 'resolving' when it's really a failure for it to protect the output from something as simple as a change in one short run of cable vs another.