It is because metals conduct heat very quickly. This is the only reason. To do cooking in less time metals are used. Aluminium is often used as cooking utensils as the coefficient of thermal expansion is very high as well as it conducts more heat. And it's very old custom of using metals as cooking utensils the ancient Indus Valley civilization people used bronze metal as utensils which is an alloy of copper and tin. Because bronze is corrosion resistant. Earthen pots were used in medieval times other than metals which used to give good taste to food cooked in pots. But pots takes so much time to conduct heat and cooking in Earthen pots takes more time and so as time passed the use of Earthen pots lessened and use of metals filled the space.

2 reason not only 1

1. Aluminum, copper and many more metal are good conductor of heat which allow the metals to be used as utensils.

2. Secondly, some the metals like aluminum are cheap and durable.

3. Utensils of different shapes can be obtained easily due to their property of deformation without losing toughness.