Wires come in all shapes and sizes. If they are attached to a 200 ft pylon, they are not normaly a problem to see, but for example a single strand telephone wire from a telegraph pole which blends in with a hedgerow may be very difficult indeed to spot, especially in low light levels against a poorly-contrasting background.
Also, you may get two lines of wires close together, a big one and a small one. The big one naturally draws the eye, and you think you have clocked the hazard, but you miss the small 20 ft poles adjacent to it.
Any wire is going to spoil your day if you havent seen it.