Because any addictive thing is on a spectrum and you don't want the people with incentives to go all the way up the spectrum to be the operator of the harmful thing, a là the dose makes the poison. (See alcohol, cigarettes, any class of drugs etc.)

Some things are just wholly harmful where no small amount provides any benefits, some are harmful in large doses, but not so harmful in small doses that they cannot provide other uses.

Governments (ideally) don't exist to maximize profit so they are (generally) not incentivized to increase the addictions of their populace.