Why power transmission lines voltage regulation is referenced to the receiving voltage not the sending voltage?
The user at the receiving end of the line want to see a constant as possible voltage. So regulating in respect to the receiving end is the correct thing to do. That way the voltage at the receiving end becomes independent of the line load.

Compare this to a 4 wire power supply where the sense wires (almost no current and therefore no voltage loss) are connected to the end of the supply wires (voltage loss depends on the load). That way the voltage at the user or receiving end can be regulated by the power supply to be constant at the receiving end.