Interesting. My guess would be that some folks are using Splinterlands cards as a store of value. With lots of exchange wallets currently closed to Steem and uncertainty about the future, at least you can park some Steem in cards and then convert them to Hive at some point following Splinterlands migration.

I know cards can now be sold on Ethereum too... but I haven’t looked at it enough to know if there’s actual trading & liquidity going on there.

I think selling for Steem right now would be risky, unless you see a clear path to quickly convert to another asset through Steem Engine.