Mainly because the main long term cost for batteries is wear due to use. Expected lifetimes for cars match their expected lifetimes relatively well, so if car b

Mainly because the main long term cost for batteries is wear due to use. Expected lifetimes for cars match their expected lifetimes relatively well, so if car batteries are used at scale to feed power back to the grid, the probability increases that the battery will need to be replaced. (And sooner). Then there are the efficiency downsides. Storage "at the edge" means more transmission infrastructure is needed, on average, compared to more centralized storage. Also, grid batteries can be optimized for number of charge cycles, not for charge amount per unit weight. Finally, servicing individual cars is probably more expensive than replacing batteries in a storage facility. And this is before going into inconvenience aspects, such as risking that your car doesn't have a near-full charge when you need to go on an unexpected trip, or the hazzle of handling the payment agreements, etc, just for a couple of dollars per day in potential revenue. In computer terms, it's kind of running mining software on your computer GPU at night. For those especially interested, it may be fun, but for the average consumer, probably not worth bothering with.