I think the early alloy wheels were uncoated, and keeping them free of corrosion and brake dust was a constant chore that generated a whole industry of compounds. The maintenance many people believe alloy wheels still need is just a persistent hangover from those days.
Just leave a bare piece of aluminum in a salt solution for a while and see what it looks like. Or take a look at some unmaintained old coated alloy wheels.
I patched up a set rather than pay more than they cost me, to have them refinished. I assure you they looked like raw metal, but had a thick clearcoat on them. Where the wheel weights attached, the clearcoat had been cut, allowing moisture and corrosion to spread under the clearcoat.