It's just not that simple, unfortunately. The "brain"'s of shared vehicles are wired into the ignition system and perhaps even integrated into the controllers, and they came that way from the factory. An engineer at the shared vehicle operator configured the firmware to talk to the company's servers. When those servers are offline, it takes someone who knows what they are doing to get them to turn on with a switch, key, Bluetooth signal, etc. Paying those people would cost more than you'd ever be able to sell the vehicles for.
Once you get them to turn on, now you have a much bigger problem. Shared vehicles get treated like rented mules. While most users don't run into mechanical problems that often, that's because there is a full time staff of trained mechanics who are constantly fixing them, and a lot of those seemingly rock-solid vehicles are practically held together with chewing gum and silly putty. As a user, you don't see the dozens or hundreds of vehicles at HQ all torn apart, or the corner of shame where all the unfixable ones go. If you allowed the public to buy these things without the staff support, buyers would get hurt and the company would be liable.