The problem is getting people to actually agree to plug in their car and let the power company run it. That's a huge ask to get people to agree to, it's going to take serious compensation that power companies won't want to do.
Why not? Where I live, consumers already get compensated ca €0,08 per kWh generated by their solar panels. At peak times, where spot prices are sometimes €0,3 or more, they could pay extra for people who want to provide power from batteries.