I think the reason for the proliferation of chain emails is the relative ease with which they can be forwarded. It takes literally seconds to type addresses, click "send," and stave off several years' bad luck. It's superstition combined with laziness and a lack of consideration for others that fills our inboxes.
Compare that to 15 or more years ago when chain letter envelopes were left on church pews for the gullible reader to find. Forwarding them was a chore, requiring photocopying (I can only imagine re-typing with carbon paper), envelope stuffing, and finding a church pew or pigeon box to leave them in.
Old school chain mail required a little bit more work, time, and money to follow, which is why it didn't proliferate as much. It was not so much rationality at work as it is laziness and the lack of willingness to spend.
Nowadays it's only too easy to be an idiot.