Aluminum can be polished to a mirror finish, just not after it has been coated. The aluminum gets a coating because, as careful said, the base metal is quite soft and easily damaged both superficially (bore polish) and structurally (straightness/ roundness).
The typical coating method is anodizing, which is an electrolytic/chemical process of oxidizing the surface of (in this case) aluminum. Translation - it uses electricity to grow aluminum rust on the surface. The process does not produce perfectly even results so even if the surface starts out perfect, it will not remain so throughout anodizing.
Some companies (like JBU) have sold barrels with a teflon finish, which simply means the barrel was anodized in a bath containing PTFE so the final surface has a film of dry lubricant as in inclusion. While I get what JBU was going for, I can't see any advantage of going with a barrel made from a softer, unalterable material over the improvements that can be made on lower priced brass/steel option.
