Psychoactive substances regulations exists to give government some measure of control over what substances people use, how they use them, and who uses them.
Cannabis is not regulated. It is prohibited. Paradoxically, in the case of cannabis, prohibition means that cannabis is almost entirely uncontrolled. Most people who want to use cannabis does so, including minors. For minors, cannabis is as readily available as alcohol.
Ostensibly, the purpose of cannabis prohibition is harm reduction. The three pillars of harm reduction are supply control, demand reduction and problem limitation. Under prohibition, there is no control of supply of, no reduction in demand for, and no limitation of problems caused by, cannabis.