In principle, the constitution outlines a federal government with specific, limited, enumerated powers. Over time, our interpretation of what those powers are and how they can be applied has changed a lot.
The 18th was considered to be needed because although Congress does have the power to regulate interstate commerce, obviously selling alcohol isn't really interstate commerce. You might be brewing your own beer, or buying whisky at a local bar that was distilled in the next city over. Since it's not interstate commerce, it's not one of the enumerated powers, so Congress can't do it without an ammendment.
No ammendment was considered to be needed for marijuana because, by the time the issue came up, our understanding of the commerce clause had changed. Today we would say that everything is interstate commerce. Even if you grow it yourself, you could choose to sell it across state lines, and your decision to grow versus buy was probably impacted by the price of buying it, which in turn depends on the price in other states. In Wickard v. Filburn (https://en.wikipedia.org/wiki/Wickard_v._Filburn), the Supreme Court found that growing wheat on your own land to feed to your own chickens was interstate commerce, because his decision to do so had an economic impact on interstate commerce. In Gonzales v. Raich (https://en.wikipedia.org/wiki/Gonzales_v._Raich) the court made it clear this applied even if there is no (legal) interstate commerce.
Under these precedents, the 18th amendment would not be necessary. Whether that suggests the poor benighted fools back then just didn't understand their own constitution, or whether modern jurisprudence has abandoned the true meaning of the text, or whether the constitution is a living document that may mean different things in different eras, even if the wording doesn't change is a matter of heated debate. :)