Why is the heating element in an electric kettle placed at the bottom and not at the top?

This is due to the phenomenon of convection. The heating element of the kettle heats up during operation and heats up the layer of water immediately adjacent to it. Warm water immediately rises due to convection, cold water comes in its place. Thus, during the heating of the kettle, all the water in it due to convection is constantly mixed and heated evenly. If the heating element were on top, then the upper layer of water would already boil, and the lower one would remain cold.