Sure, we have receptors for very microscopic quantities of various opiads, but the aggregate design is not to consume mass quantities like some cone head sucking down eggs and beer.
Now, regarding alcohol, it's been found in intersteller space. MJ hasn't.
No doubt if it is found you'll want to go there. We will contribute to your trip!
Cannabinoid receptors react with only one set of chemicals produced by one variety of plant life on the whole of the world. Opate (opiad sounds like some sort of athlete high on heroin trying to do the pole vault) receptors work with a myriad of substances, man made, plant made, and chemicals made within the brain itself.
Interstellar alcohol - big effing whoop. It's a simple carbon-chain molecule, no doubt found in great abundance in the universe.
Now as for potency of cannabis - while it is true that there is no way to make THC more potent, it is possible to grow a plant that produces more THC glands, making the cannabis more potent in THC. Many a pothead has studied botany throughout the years and a great many strains and hybrids have been developed to produce more THC pound to pound against your "Mexican MJ" which may have been the "good stuff" when you were back on the block, but now it's high-yeild high-potency stuff coming out of middle aged white-guy's attic.