Answer:
Because that's what it is. It is a voltage divider.
Two resistors in series will have the same current through them. This is Kirchoff's current law. By Ohm's law, then, the voltage across each resistor is current times resistance, and this is a linear function. By Kirchoff's voltage law, then, the total voltage drop across both resistors is equal to the input voltage.
Two resistors of the same value will divide the voltage in half. One resistor (the top resistor) having twice the resistance of the other, will divide the voltage to a third. If the top resistor is nine times the bottom resistor, the divider produces one tenth. And so on and so forth...