Are there two types of resistance?
The potentiometer heats up too, it's just that this isn't its intended purpose.
This is, in fact, the reason you don't use a potentiometer in the way you've described. If you were to actually hook it up like that, it's quite likely that either the pot would overheat, or both the pot and the heater would remain cool. Instead, the proper way to do this would involve using phase angle control with a thyristor for AC, or PWM control with a FET for DC.