Some larger modern bikes have linked brakes. On many bikes, the front brake is a hydraulic disc or two, and the rear brake is a cable actuated drum, which pretty much necessitates separate systems. Not to mention that bikes experience far greater weight transfer under braking than a car, and locking up either wheel can be very bad, so having independent control of them can be safer. Bikes with linked brakes tend to be big bikes that are extremely unlikely to lift the rear wheel, and may also incorporate some form of ABS/electronic brake control.