Your question is conceptual; so it can be answered by the help of well-known simple electrical concepts as voltage divider, potentiometer, rheostat...
Logic gates, like most digital and analog circuits, are devices with voltage inputs and outputs, ie, they are voltage-controlled voltage sources. Their output stages are implemented as voltage-controlled voltage dividers consisting of two elements in series. One of them (pull up) is connected to Vdd and the other (pull down) is grounded.
The main property of these "pulling" elements is resistance. But while in the simple resistor voltage divider their resistance is linear, in the output stages of electronic circuits it is nonlinear (dynamic); this causes them to switch quickly.
The output voltage of this voltage divider configuration is controlled by changing the resistance of one, the other or both elements (in opposite directions). For this purpose, they are implemented by dual transistors (MOSFET, BJT, etc.).
The output stage could be implemented by only one controlled element (transistor) connected to one of the supply rails only if a load with relatively low resistance is connected to the other rail. This configuration is called "open drain (collector)". Actually, the transistor and load constitute a voltage divider as above... but this is not the case here...
So, according to simple electrical concepts, in the case of a low-impedance load, we cannot change the output voltage with only one resistor (PMOSFET in your circuit) connected in series to the supply rail since there is no current flowing through and voltage drop across the resistor. We need another resistor (NMOSFET) connected to ground to provide a current and voltage drop across the upper PMOSFET.
The load can serve as such a resistor if it has relatively low resistance. If not, we have to connect another resistor to ground, in order to close the circuit.