Every car hack I have ever seen has depended on either
1) Physical access to the car, that is sticking a new device into the car's network. That is time consuming and not at all inconspicuous. Defending against such attacks would be a good thing, but if you want to sabotage a car and have the opportunity to open it up and tinker with the electronics, attacking the computers is probably the hard way to do it.
2) The manufacturer not paying any attention at all to security. Safety-critical components should not be talking to non-safety-critical components at the very least. This is understandable, because they have only recently started needing to. They will get better as the need becomes obvious.
Security is something car manufacturers need to start paying attention to, but car exploits aren't something to panic over. Not yet, at least.