You're right, there are no logical reasons, but there are plenty of other reasons why this essentially benign weed should be kept illegal.

Hundreds of thousands of people have had their lives ruined because of marijuana arrests. Although some were actually suspected or even real criminals and the dope was just a convenient excuse to get them into custody, the vast majority were just normal people, mainly kids, minorities, or the poor.

Tens of millions of other marijuana users got away with this crime, and many of them are now in positions of power and authority in the society, or are otherwise productive and law-abiding citizens. Most of them continue to use marijuana with no ill effects.

The moral, legal and political authorities who have been behind this witch hunt do not want to be faced with the responsibility of admitting that they were mistaken and that this expensive and brutal oppression has been carried out for no good reason. Essentially, they have made an easy and lucrative career out of persecuting pot and they don't want it to end, and they don't want to face the responsibility for their crimes.