Why can’t I just buy a battery pack, or make my own from deep-cycle batteries, add a small fuel efficient generator, and the interface hardware, and make a couple grand a year?
Why pay for the rest of the car if this will be profitable to me?
I suppose you could, but the dedicated capital cost may make that idea unprofitable. The apparent attraction of V2G is that it makes use of capital assets which would otherwise be idle: excess night-time generation capacity of the utilities, and excess storage capacity of the vehicles.