Probably the best answer is that we do in fact control the voltage applied to the element by turning it on and off. This is not the instantaneous voltage but rather the rms voltage. Applying 220 V at a 50% duty cycle is applying an rms voltage of 220/sqrt(2), for example.
We are concerned with controlling the temperature of the load (the mash), not the temperature of the heat source except broadly in that we don't want it to go to such a high temperature that scorching can occur.