Why are saucepan usually made out of metal?

In general, metals are tough, useful materials that are, in general, fairly good conductors of heat. Metals, for the most part, will not be deformed by the heat of cooking, and, because they conduct heat across their surfaces, they can be used to cook in.

If, for example, and aluminum pan is used to prepare something like, say, scrambled eggs, we apply the heat to the bottom of the pan, add our eggs in a short time, and they'll cook up nicely. The heat that is applied to the bottom is fairly evenly transferred all across the bottom of the pan, and the eggs cook fairly evenly. Additionally, the modest amount of heat required to cook the eggs will not cause the pan to be adversely affected. That pan holds its shape and maintains its strength quite well. This makes the application of aluminum as a material to make pans out of a good idea. Certainly other metals can be used, but the same basic principles apply. It's just that there are trade-offs when using aluminum, stainless steel, copper, cast iron, or anything else.