Because it is a plant, It occurs naturally, it is not a manufactured product like a Gun or Sword
So to follow this argument "logically" cocaine and crack cocaine, materials produced from the cultivation of coca plants shouldn't be controlled? (outside of the trade in them you mention below)
Please provide link for where I can get seeds for the crack cocaine plant. Is it in the same aisle as the crystal meth tree?
Seriously, there is a distinction between the natural product (The Leaf) and the highly processed street product.
The natural leaf is relatively harmless and actually provides protection against mountain sickness which is why it has always been taken in the Andes.
The highly concentrated, and indeed chemically altered, street product is a different thing altogether.
In principle a home grower could concentrate their own cocoa leaves, but there really wouldn't be any point. The only point is for transport and trade. something that is already illegal and would remain so in my hypothetical world.
Why is it illogical? If a substance is to be banned or controlled then making laws against people growing it would seem to be fairly prudent and rather logical. Seems rather flawed to try to control something but then leave in place a massive loophole.
Again, that Horse and Cart thing.
If it is illogical to make growing the wrong sort of seed or eating the wrong sort of food unlawful, then it is also illogical to make the products that are contained therein unlawful.
The control is in the control of trade. the trade flourishes because it is profitable. I accept that prohibition of trade is only of limited effectiveness but I do not see replacing the Medellín cartels with BAT as being a good solution. Allowing those who wish to to grow their own to do so while at the same time maintaining restrictions on trade would go a long way towards damaging the money flow in the illegal trade.
The biggest opponents to permitting home growing is actually likely to be the illegal trade. They would view allowing people to home grow in the same light as Microsoft would regard somebody writing a Linux distro that could seamlessly run Windows applications.
Discussions like this always end up referring to the US 18th amendment as the prime example as to why prohibition doesn't work. Its almost a sort of Godwins Law for drugs talk.
The problem with the 18th amendment is that it sought to ban alcohol altogether. This wasn't actually what most of the temperance campaigners had in mind. They sought to ban distilled alcohol but would have been fine permitting naturally brewed drinks.
Had they restricted the Ban to spirits, or at least permitted home brewing of Beer and Cider and wine, then there would have been a lot less public resistance. Far reduced opportunities for organised crime and the 18th amendment might well have survived to the current day.
Edit to add.
In support of the above assertion, it is worth noting that many parts of the US have
local prohibition laws in place to this day
See
https://en.wikipedia.org/wiki/List_of_dry_communities_by_U.S._state#Kansas