The reason why it is easier to filter by wavelength is that almost any optical path reflects significant amount of light back, primarily from various discontinu

The reason why it is easier to filter by wavelength is that almost any optical path reflects significant amount of light back, primarily from various discontinuities along the fiber (connectors, splices, and also the actual receiver on the other end) but also from imperfections in the fiber itself. On copper interconnects there are similar issues, but for reasonably controlled cabling (eg. 1000-base-T) they can be minimized enough, that what remains is either insignificant or can be compensated for by DSP magic.