It's crazy to me that landlords don't need to be licensed. I can't think of any other service provider that provides anywhere near this level of accommodation that doesn't need to be licensed. I know there are a handful of cities that have it, but it should be done federally IMO.
It would help make sure the accommodation is to code (fire escapes, alarms, mold, locks etc.), as well as ensure that the income collected in on the books and taxed properly. It would also ensure the landlord knows the laws, rules and guidelines they must follow.