Why A transistor is a three-terminal active device in which a small current or voltage at the base (or gate) controls a much larger current between collector and emitter (or drain and source). Driving it hard on (saturation) makes it look like a closed switch, and driving it off (cutoff) makes it look like an open switch, with no moving parts. That same control action is what gives a transistor gain when it is biased in between those two extremes.
Watch out A potentiometer is a manually adjusted variable resistor, not an electronically controlled one, and a varistor or thermistor simply changes resistance in response to voltage or temperature rather than to a control signal.
Only the transistor has a control terminal: base in, switch or amplify out. The -istors just change resistance on their own.