Why Is a Ground Wire Used in RS-485 but Not Needed With Ethernet Differential Signaling?

I have been asking this question but have not been able to find a good answer for it.

When we have differntial signaling, we don't need a ground reference anymore. And that is becuasae the differntial part is acting as the ground.
But then I noticed that in different application when we are sending differntial signals from one unit to another we somehow care about grounding, and I am not sure why?

For example, in RS-485 transactions, we normally connect a third wire as a ground reference.
And in other forms (for example ethernet), we use isolation transformers for galvanic isolation.

Why do we need to send a third wire (ground) in RS-485 commmunications?
And coming out of ethernet PHY, the signals is already differential pair (and to me that is well isolated, since there is no need for ground referecing). Then why do we even care about isolating it even more by the transformers?

I though that you could care less about ground reference differences when you are dealing with differential signals !!!

Ground reference is necessary to prevent common mode voltage or signal bias voltage from exceeding the receiver limits. Cable shielding can provide that reference unless the driver circuit uses isolated power. Then a reference ground wire needs to be connected to prevent corrupting the differential circuit.

Bottom line 422 & 485 signaling circuits are not perfectly isolated from power and signal ground. External environments can cause bias or floating voltages that cause receiver error.