Why is marijuana illegal?

smoking marijuana is proved addictive. but test have showed that it is less harmful and better for u then say cigs other additive drugs and alcohol. i dont understand why we just tax the hell out of weed and sell it in some secure case, like i dont know where they keep cigs and alcohol in stores. They did some studies in South America between infants whose mothers had smoked heavily and mothers that had not used marijuana at all. The infants whose mothers smoked heavily scored better across the board in terms of development factors. some things the government just dont want us to do though right.