You can't provide manual override for electronically controlled system in the case of power failure. To disengage an electronic parking brake, at least in Tesla

You can't provide manual override for electronically controlled system in the case of power failure. To disengage an electronic parking brake, at least in Teslas, you need power. If there's a serious problem in power supply (of 12V power), the computer must put the car in the likely safest configuration and shutdown to prevent any kind of erratical behaviour. Anyway, in an unlikely situation that the driver fails to pull off safely in case of critical system failure (remember, steering and brakes always work), and the car stops on lane, you probably don't want to risk your life by pushing the car. For a towing truck the parking brake is not much a problem.