Copper is used in strings, but as an alloy. Since about, oh, 4000 BC, metallurgists have known that, by mixing different metallic elements together, products such as bronze (a simple alloy of copper and tin, but often containing other elements such as phosphor) could be produced that had characteristics desirable for the job at hand. As a matter of fact, bronze is less brittle than iron (but note, this is not true of steel, of which iron is a constituent).
Now, let's just do a thought experiment: you are a string manufacturer, and you want certain qualities that seem to derive from having varying amounts of copper in the alloy.
So do you A) tailor your new mix so that your strings, when constantly flexed by tuning or vibrated by playing get hard, brittle, and stiff? Or do you B) tweak the mixture to minimize those traits and thereby make them (get this) not WORSE for use as guitar strings, but better?
I won't spoil your fun by providing the answer.