If I understand correctly, the answer is "they probably did need to, but didn't bother".

You'll sometimes see articles say "these states have legalized marijuana, but it remains illegal at the federal level". If the DEA did choose to try and enforce marijuana prohibition in one of these states, the state would be able to sue, making the argument that the US constitution does not give this power to the federal government. The federal government would probably argue that it did have this power under the Commerce Clause.

My understanding is that the DEA has avoided putting itself in a position where this could be litigated.