Why does the voltage decrease across resistors in series?
We think in ideals. 

In the ideal circuit shown below, when you generate a voltage, with say a battery, then the applied voltage across R1 is the battery voltage. Further, your implication that there is no voltage drop across the resistors is incorrect. There is a voltage drop, it is equal to the entire applied voltage.

As you can see, attaching a second resistor will not change that voltage.







simulate this circuit &ndash; Schematic created using CircuitLab

However, in reality nothing is ideal. In the real world the battery has some resistance as do the wires between the battery and the resistors as shown below.







simulate this circuit

In a good circuit R_Source will be very low but is finite. As such there is some voltage drop across it. Now when you add R2, the total current through R_Source increases and the voltage across AB will indeed be lower.