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.
https://en.wikipedia.org/wiki/Gonzales_v._Raich
As for why an amendment was needed in 1920 but not 1971, apparently the commerce clause didn't become the anything clause until after 1937.
If this all sounds hopelessly inconsistent and/or hypocritical, well, I would agree with you.
I agree with your sentiment that the current prison system is a major problem.
https://time.com/4080110/dea-medical-marijuana-california-ru...
There is so much federal policy based on commerce clause jurisprudence that overturning Raich and others is a pipe dream. Change will only come from slow erosion by giving the Supreme Court the opportunity through some creative cases.