A potentiometer is an adjustable voltage divider. It has both resistors (actually only one resistive element but with the wiper dividing it into two resistors).
As for the LDR, I don't agree. I've only seen them used with another resistor. Maybe you can provide a citation for your assertion.
Of course the microcontroller on the Arduino does have internal pull-up resistors, which you can turn on/off via pinMode().