Originally posted by Sparkiekong:

It's essentially a slow down mechanic. If you think of it logically, it makes no sense. You can go through carrying and wearing tons of metal, but you can't take through processed or unprocessed metals? It's designed to make you utilize the boats and wagons so that you explore and see more of the game and experience more.

You're using the wrong logic. Ores and metals are devoid of life energy, while other non-organic materials are very porous to life energy. Manipulating 'stuff' imbues it with the manipulator's life energy to a degree depending on how porous or resistant the material is. It's perfect 10th world logic.