Voltage Divider, \$V_{out} = IR_2\$
As you have already said, the formula gives you the voltage drop across that resistor.  So it is the difference in voltage between one side of the resistor and the other.  As this resistor is the "bottom" half of the divider the voltage at the lowest end will always be 0V, because that is where it is connected to. It can never* be anything else.  So that means that the voltage at the other side of that resistor must equal to the voltage drop across the resistor - it can't be anything else.  And as that point is also \$V_{OUT}\$ then that's what your output voltage will always be.



*as long as you measure with respect to that point as ground