Ask HN: Is ultimate car sharing possible through just the ODB port?

I am not very exactly an expert on cars, or apps. I would say im average in both so when this thought stroke me I thought I should ask the community of smart people, the HN.

Since all cars (made in last 20 years as far as I know) have ODB ports and since you can almost completely control the car through the ODB port, whats preventing a startup from creating a device that would plug into your ODB port and connect to a cellular network and when paired with owners phone would enable him to seamlessly share the car. If its possible to lock/unlock car, start/stop engine through odb port, the plugged device equipped with gps chip and cellular antenna would enable the owner and the company to control the car remotely from anywhere with cellular coverage. the company could vet members using their credit scores, insurance information and drivers licenses to make sure nobody takes advantage of the trust put into the system by car owners. Its just a thought and I probably miscalculated something, please feel free to point where Im wrong.