It would make sense to make them out of metal because it more resistant to the elements than wood. Also it takes a lot less time to just bolt the metal frame and metal panels than to build it out of wood which would take a lot more time and effort. Also if you need to expand a metal shed you just remove the end and bolt a longer frame into it rather than having to start again with wood.

Metal Buildings are much stronger and more economical.

Many developers are increasingly choosing to construct their buildings out of metal for various reasons. Metal buildings are stronger and less vulnerable to the ravages of time and weather.