They aren't really 'rare' they just can't be cultivated in captivity at a commercial scale well due to their need for a symbiotic relationship with trees as well as Morels being very picky about their fruiting triggers. I have seen people successfully 'seed' an area by making a slurry by blending molasses, water, and wild foraged morels that were subpar and letting the spores germinate with a water aerator running in the bucket and dumping the slurry in areas on their property that were favorable to Morels.