Why some through-hole component leads are ferromagnetic?
Main (and probably only) reason: copper is expensive. I've commented before that cost can outweigh other factors in design and production, and that goes also for components. Every milli-cent counts. The alloy used (no, I don't know what it is) may have a higher resistance than copper, but over the whole the difference will be negligible. I guess that for very low value resistors (&lt; 0.01\$\Omega\$) copper may be used. Copper is also used for certain power devices because it conducts heat better. These 500\$\mu \Omega\$ examples from Isabellenhütte illustrate both: