That is because aluminum flakes as it degrades. And aluminum flake burns, just like magnesium flake. Why do you think it is one of the components of thermite, or at least the home made version.

I asked around, and while I don't have the engineering experince that some here do, some of my coworkers do. They all said flat out that the reason why people pushed aluminum wiring was because it was lighter and cheaper. They looked at my like I was nuts when I asked if it had a lower resistance.