I suspect the metals are oxidized on the asteroids, too. If aluminum takes so much energy to liberate from the oxygen, it must really like to combine with oxygen. I'd guess there was a fair amount of oxygen floating around with the aluminum before it clumped together into asteroids, even if there isn't now.
Asteroidal resources are unique and diverse. In general, they are different from the dirt beneath our feet and the ores that we mine from our planet's crust. Likewise, they are much different than the Moon, Mars, and other planets' crusts. Asteroids are much more metal-rich, like the core of the Earth, Moon, Mars and any other planet, and unlike their crusts. The asteroids are normally not as enriched in oxides, silicates, and the other lighterweight elements that floated up to dominate a planetary crust. Many asteroids are like a mixture of a planetary metal core plus a crust plus the mantle in between. Some are entirely like a core or a mantle or a crust, having come from a large parent body which was broken up. In most asteroids, free metal is abundant in the mix, and often dominant. This free metal can be used without any further industrial processing.
I don't have a good explanation why (lack of pressure, maybe?) but this doesn't seem to be the case. For example, I've seen the 60-ton Hoba meteorite in Namibia, and although the surface has corroded and oxidised, its bulk is definitely solid metal (mostly iron, some nickel).
It would still take some processing to separate the compounds, but nowhere near as much as ores.
