Transmission line losses are proportional to the square of the current (resistive heat losses in the wire). Current also requires larger transmission line cables, and hence stronger towers, driving up the materials cost.
You can trade current for voltage, so long distance transmission lines have very high voltages, thousands or tens of thousands of volts. That keeps the transmission losses down. Actual losses tend to be more in the 2-13% range (mostly around 6%) in total - the long distance transmission lines generally account for about 2% and the rest is local line losses