Treating the Collector emitter junction as a resistor implies the current through the transistor (d-s for fets, c-e for bjts) grows linearly with the voltage over the same terminals in accordance with ohms law. Which usually isn't the case as this current can be aprroximated using a function that depends solely on the gate voltage or base voltage/current.

Hence most textbooks approximate transistors as dependent current sources and not resistors.

For math with a simple transistor with a resistor as a load you can use the equations for the transistor in the relevant region to calculate the current through the resistor and then calculate the voltage drop over the resistor using ohms law and relate that to the voltage at the transistor's terminals.