Because you can already buy them. They are called GPS trackers. Try a Google search for "Car GPS Tracker".
It can't be hard to do it better. And if done right there's a massive market. You could get in with insurance companies to cover costs like NEST did with the energy companies. The Business model writes itself.
No GPS built in, so you could either add a GPS module or hope that cellular/wifi triangulation is good enough resolution.
I agree this feels like something that should be available off-the-shelf at a reasonable price by now.
Why?
LoJack is pre-GPS. There's a tiny unit hidden somewhere in the car, connected to vehicle power. Normally it just sits there, transmitting nothing. LoJack has deals with major police departments. When a vehicle is reported stolen, their system picks up the stolen car report, and sends a signal out on a subcarrier of commercial FM broadcast stations, with the ID of the LoJack unit involved. The LoJack unit picks this up, and starts broadcasting its beacon signal, using car power if available and its own battery if necessary. Police cars equipped with LoJack receiver gear (four antennas in a square on the roof) light up a display with direction, approximate range, and info about the target vehicle.
Recovery rate of LoJack equipped vehicles is about 90%. LoJack recovers about 10,000 vehicles per year. It's a 30 year old, effective technology.
I don't know about US market, by in my country an anti-theft GPS trackers sold for about $100 (and about $50 for installation). Not a lot compare to a vehicle cost. Also your need a SIM card (to send/receive SMS by tracker). Even in the US it is not expensive (e. g. T-Mobile for $3/month).
Of course, it comes with its own downsides. For instance you'll lose reporting if they pull in an underground garage. On the other hand, if they ship your vehicle to the South Pole you'll continue to have reception.