Copper cables, in their various forms, have been the de facto standard for interconnecting network devices since the beginning of business-related computing (perhaps even longer, if we count the cabling systems of telephone service providers).
Copper cabling was an almost intuitive choice in early networks, which were mostly peer-to-peer topologies. It's relatively inexpensive, readily available, easily installed, and has low resistance to transmitted electrical signals. On the other hand, it has a limited transmission range and is susceptible to external electrical interference or noise.
When properly installed, copper cabling is both reliable and easy to maintain.