Nor should they. People don't want to be cycling their batteries and reducing their life. This use case would be better served by batteries that are designed for that purpose instead of being designed to be light for a vehicle.
This is how most Time-of-Use metering already works. The driver sets a minimum battery percentage to charge immediately (eg 40% range, enough to reach the local hospital), and then schedule a time when the car should be fully charged (eg 80% by 7AM). The software just Does The Right Thing, using the same prediction and bidding algorithms as stationary batteries.
The search term is "V1G" (a cheeky reference to unidirectional V2G).
More battery cycles just costs money. For the right price, I'd do it.
But more than that: I don't want to be stranded without power in my vehicle because someone in the electric grid made poor power management decisions and decided to offload that decision to consumers.
That said, most EV incentive programs use around 10% (often less) of an EV battery capacity so the actual effects are barely noticeable.
A price that can be measured in money. How much more does it need to pay to be worth any extra hassle?
So in a sense it is just money. Money is hassle, fundamentally. It's a hassle to make it and you spend it to save other hassles.
Another factor is that not all charge/discharge cycles are the same. Going between 60% and 80% five times is a lot lighter on the battery than going between 0% and 100% once. Which pairs great with EV batteries, because their batteries are deliberately oversized compared to average use to account for uncommon events.