I go hiking all the time and have never had an issue, and I don't worry much at all.
Thieves are usually about crimes of opportunity; meaning a quick and easy opportunity for theft just happens to present itself. Driving out to the middle of nowhere, and hoping to come across a vehicle, and then hoping that vehicle might have something of value in it (worth more than enough to cover the time and expenses [gas] to make it out there), isn't exactly a crime of opportunity.
This is why roof bike racks have the option to add locks; the locks are probably enough to prevent a crime of opportunity if you've parked in a reasonably well traveled area (running in to buy a coffee at a interstate rest area during the day? You're probably ok.); if a thief really wants your bikes and you're really careless about where you park, they'll be gone even with the locks.
I'm not saying crime/theft/vandalism will never happen if you leave your car alone at a trail head, I'm just saying it probably won't happen to you. The vast majority of people respect property and the law.