(1) There are many synthetic narcotics (eg, oxycodone) that are more effective than opiates. (2) Medical facilities pay a (relatively) low price for narcotics, at least in comparison to the street price. Since addicts cannot purchase narcotics at the lower rate, the profit margin in the illegal trade is much higher, and opium farmers can make more by moving their product into that market.

I guess that the question of whether or not addicts should be allowed to buy lower-cost, legal, narcotics is a separate one.