It's a simpler method than trying to actually move the power up and down directly.

Remember, heating elements work by Joule heating. Thus, they satisfy the law P = V^2/R, meaning that the power is proportional to voltage (V) squared, and inversely to resistance (R) (to rough order because technically R depends on temperature T). To modulate the power high to low in the way you're thinking, you have to decrease the voltage. Now given such things are powered by AC, that's not super hard - just use a transformer - but having a transformer in the appliance (and an adjustable one at that) still adds weight and complexity, and thus cost.

But here's the thing. Thanks to thermal inertia, if you instead subject the heating element to an intermittent/pulsed power input, then so long as the pulsing interval is not too large, it will heat up as though it were being subject to a continuous input of heat at a fraction of the maximum power equal to the duty cycle fraction, i.e. how long each pulse lasts versus the total time between pulses. That is to say, the heat capacity of the heating element causes it to act thermally like a low-pass filter, so the temperature response looks like a greatly smoothed version of the power input waveform. Hence if it stays on 30% of the time and is off 70%, e.g. a 0.3 s pulse followed by 0.7 s of off time, then the heating element will act like it is receiving a steady 30% of the input power (or if you like, sqrt(0.30) ~ 55% of the input voltage), with no transformer required, just a switch (maybe a transistor, but still, it's a switch). And switches (incl. transistors) are cheap and easy to make and use.