I think you're right, Cecily. They have probably been cultivated for too many years and have lost the ability to multiply the natural way. I remember seeing a large colony of wild Crocuses in our local forest, but it's probably too late now to go and find them and dig a few up for the garden. Anyway, they are quite cheap to buy, as you said, and delightful to watch during the short time they bloom
I just spoke to my neighbour and he thought they were genetically modified to NOT reproduce. Crocuses, like many other bulb plants, are a huge export article so the greenhouse owners earn more money when we have to pay for each and every single bulb. I think he has a valid point.