If you drive under a low overhang or into your garage with bikes still on the roof rack the damage is strictly your responsibility and this type of forgetful accident is probably as common as rear end collision damage.
Probably way more common! I have two friends with roof racks and both have done the garage thing in the past 6 months. In both cases they were riding shotgun in the car and the wives were driving and neither thought about it as they drove into the garage. One destroyed a $4000 bike and the other was cheaper. I know the one with the $4k bike did get something from the insurance.