Why is the voltage drop large in a high voltage transmission line?

The length of the wire causes resistive load to be distributed along line. This accumulated resistance causes the voltage drop.

The voltage drop along a high-voltage transmission line isn't large in relation to the operating voltage of the line. One of the advantages (there are many) of using high voltages is that, for a given load, the line current is relatively low -so the voltage drop along the line is also relatively low.