There's a difference between a chemical analogue and an alternative way of producing the same chemical.
Glucose is still glucose whether it comes from sugar cane, corn, beets, or other sources.
Marijuana is more than just THC.
Yes, it's a variety of ingredients, but while medically-used THC comes from a cannabis plant, it's often been purified so much that it's still only THC.
And coffee is more than just caffeine, but both are popular primarily for their psychoactive properties.