How is the capacity of link more for single network link
the wire can be either high or low

This assumption is wrong and is the source of your doubt. At high speed it's perfectly possible for one end to be high and the other low. A transition ("edge") from high/low takes time to propagate along a wire. It doesn't propagate at the speed of light either, but at a rate determined by the capacitance of the cable. 

At high ethernet speeds it's perfectly possible to have multiple bits in flight along a cable.