Although I have never used marijuana, it always surprises me when someone who has tells me, "I don't get it. Why would the government go to so much trouble to stop me from smoking this? It has taught me so much about life and the world." Well, there you go. You've answered your own question.
The government is basically a giant gang ruled by super intelligent psychopaths, basically. The same is true of corporations by the way, and also religious institutions. And the one thing that these rulers have in common is this: they are very afraid of being revealed and of losing their power.
You hear all the time about horrible things that happen in places like Syria and North Korea, where the governments are "totalitarian". Well, the little secret is that all governments would really like to be totalitarian. Places where horror stories emerge are merely instances in which the governments actually succeeded.
So how did you answer your own question? People like you keep telling me that this drug is making you realize that things are really unfair and that we ought to change it, do something about it. We all ought to love each other and treat each other more fairly. Sure, ok, I agree. The problem with this is that power hungry psychopaths do not want you ever to come to that conclusion. And if they find out that something as simple and stupid as smoking a plant is what caused you to realize that there was massive exploitation, they simply fight the plant.
Think about it like this. Say you are a cattle rancher, and you find out that when your cattle eat a certain weed, they figure out they are being farmed for slaughter. What do you do? Well, that's simple. You stop them from eating that weed.
So is it really that hard to imagine that the government would spend more money than it would take to feed all the hungry people in the world simply to make you stop smoking that plant? No, it actually makes quite a lot of sense. You just have to understand a psychopath.