Since they don't have value and there is no profit from mining, what's the miner's incentive?
Demand for something = value. That value does not necessarily mean a monetary reward.
You can create a free bitcoin Testnet site, the value will be in the number of visits to that site.
Some faucets offer these coins for free = the value is watching some ads or clicking on some links.
Some try to sell it and others try to use it to scam.