Unless you're a major automotive manufacturer, though, a ton of your startup time and cost is going to be burned buying and modifying a car to test your software on. It can't just be any car. It needs to be fully drivable by wire, including gas, brakes and steering. And it needs to have a bunch of onboard cameras and sensors, as well as the processing power to deal with all that input.
Not to mention, not everyone wants to start teaching a car to self-drive from scratch. Autonomy is such a gigantic and complex problem that there's plenty of hay to be made chewing away at small, digestible chunks of it.