The number for copper is not a fixed quantity, it varies with the type of cable. Electric fields are outside of the copper wire, not inside. The copper conductor acts as a wave guide for the electromagnetic wave. So it turns out that things around the cable have an effect on the speed of propagation [1], particularly the insulator. Bare copper wire in a vacuum would be very close to c. In the case of fibre, the issue is index of refraction.
[1] https://en.wikipedia.org/wiki/Velocity_factor