Is there any good reason why so many miners are apparently mining it?
They must have an incentive. I can't really believe that paying the hosting of a faucet website plus the mining costs are less than the ad revenue, but they must have a profit somehow, otherwise they wouldn't mine. That profit may not be translated into money.
My best guess is that testnet is used to test custom mining software, and any improvements and/or bugfixes to mining software. It may also be used to test things intended to reduce orphaned blocks of the miner.