Ethernet uses transformer coupling to the cat5 twisted pairs. The encoding method also minimises DC offset in the signal which would saturate the transformers. This creates a near perfectly balanced signal along with the consistant twisting of the wires so the system does not radiate much, this also makes it fairly immune to outside interference. Remember that CAT5 ethernet is only meant to run 100m and indoors. In hostile areas like factories, STP is used or in many cases now, fibre optic. Most of my installations that use RS485 are outdoor and around the 1km distance. I could use fibre optic, but the cost of using fibre is still overkill at the moment. There's also the cost of installation and maintenance - you need a OTDR unit to diagnose fibre problems and these are usually >$7000! I only need to talk at 19200baud! However the time is coming when it will be economical to add ethernet to designs that previously used serial.

We use cat5 extensively in the workshop to wire up our units for testing. However, site wiring is normally done in armoured cable that is shielded.

It all comes down to the application as to what is used.