Why is an Ethernet cable not grounded?
If you just ignore the POE 48 Volts in the image below, you can see Ethernet uses transformers on both sides. 

This way there is no need for common ground as long as the common mode voltage stays below 1500V generally. The isolation specification of the transformers.



And as a bonus you now also know how POE works. (802.3at) 

However, CAT6A often has a shielded connector. The shield is then grounded to chassis using the little flaps inside the socket.

Source image