That's not quite true...
Digital cables (optical or coax) carry the digital information using an analogue physical representation; the intensity of light, or a voltage on a wire.
Both of these representations can be degraded by transmission down their cables.
Cheap optical fibres are plastic, and the attenuation per metre isn't trivial. Since they're not monomode optical fibres (as used for long-distance, high-speed optical data comms), the pulses will also smear. Make that cable long enough, and the receiver will eventually be unable to recover the data at the other end.
Similarly, for coax cables, a poor cable will cause losses and reflections due to impedance mismatches, both of which will degrade the signal, and eventually cause problems for the data recovery at the receiver.
Even if no data errors occur, poor signal integrity will increase the jitter in the recovered clock.
That said, for the short distances usually involved in connecting optical or coax SPDIF sources to SPDIF DACs, rather too much fuss is made about the quality of cables... You certainly don't need gold-plated connectors on your optical fibre cable (as I have seen suggested...), and I'm quite happy using 1m optical cables bought for £1.