There's a splendidly succinct explanation on many faucet websites of why testnet & stagenet coins shouldn't acquire value, but I'm asking how.
My first guess is that Monero's twice-yearly scheduled protocol upgrades (“hard forks”) somehow ‘reset’ things, but that just raises awkward questions about how mainnet doesn't get ‘reset’. Obviously I must be wrong, because if there were a ‘reset’ mechanism, the market would be crazy to bestow a $843 334 892 marketcap upon mainnet. Is it something to do with the hard-coded seed-node list (which afficionados of certain other crypto-assets like to think of as Proof-of-Work's dirty secret Unique Node List)?