The aluminum surfaces which I've seen 'oxidize' are generally 'cast surfaces', and possibly the somewhat porous nature probably causes the oxidation over time and with exposure to moisture and temperature changes. Prepping and painting those surfaces prevents the aluminum oxidation and promotes easier cleaning.

A 'machined' aluminum surface isn't as likely to oxidize, and is easier to keep clean or polished than a cast surface.

PS. I've used 'argent silver' urethane paint to paint aluminum parts which appears 'aluminum like' and is much easier to keep clean.