Pretty sure they could be made out of metal, but then would be heavier and less efficient.
This is something we still need to figure out - which technology works best in the long term, especially when considering foreign dependencies.
In theory we could build wind power plants and solar ones even from mostly steel and aluminium. In case of solar that requires entirely different technology - not solar panels but solar collectors that heat water using sun energy, and then processing like with every other heat based power plant - in case of wind it is basically the same, just heavier.
But even if we don't, the mere fact that we could makes us less dependent. Although it would be even better if a small number of these are actually built so in an emergency (like China stopping even supporting existing plants) we have a proven plan B.
