Because the pilot is almost never looking at a map on final approach. He is looking out the window, and in the dawn/dusk light, or with a setting sun in the background or inclement weather etc., a taxiway can look exactly like a runway.
Don't forget that almost all taxiways run parallel to the runway, for about the same length.
(NB: I used to be a commercial pilot.)
In most airports I flew into, the surface material for the taxiways was different from the runway, so sometimes the taxiway could end up looking brighter and more prevalent than the runway under certain lighting conditions. Yep, I have had moments of confusion when on long finals into an unfamiliar airport. Never to the point of actually ending up landing on the taxiway, but the confusion is a thing that pilots have to contend with.
Don't forget that almost all taxiways run parallel to the runway, for about the same length.
(NB: I used to be a commercial pilot.)
In most airports I flew into, the surface material for the taxiways was different from the runway, so sometimes the taxiway could end up looking brighter and more prevalent than the runway under certain lighting conditions. Yep, I have had moments of confusion when on long finals into an unfamiliar airport. Never to the point of actually ending up landing on the taxiway, but the confusion is a thing that pilots have to contend with.