Electric cars do have the circuitry to do this. It's built into the CCS protocol which nearly every EV speaks (US and EU).
And it's not hard to imagine how this would work. Two big connectors on EVs can ultimately be hooked pretty much directly to the battery. When you plug in, you'll hear a giant "cachunck" which is the relay switching to DC charging.
If you want to turn that into a vehicle providing power, the action is exactly the same, "cachunk" the relay which completes the circuit and have the charging station convert that DC power to AC.
That's where the actual equipment gap exists. AFAIK, it's basically only ford that's producing a V2H charger. Enphase has one announced since last year, but I've not really seen anything about it since the announcement.