It's an interesting one certainly, it sort of comes down to mechanics at the end of the day. In the event of a catastrophic, complete electrical/engine failure, the driver needs to be able to apply enough braking force to stop the car in a sensible distance using their leg muscles. This places an limit on the travel distance of the pads for a full press of the pedal.

Indeed, most cars use a servo that uses vacuum 'pressure' harvested from the engine to assist with braking and reduce the amount of effort the driver needs to expend. However, you can still brake with a non-functioning servo, all be it with a much reduced braking performance.

It's a similar reason why even very modern cars (bar a few, the Infiniti Q50 being one) still use direct mechanical linkages (heavily assisted in many cases) for steering, despite the steering rack, column and rods taking up room and complicating engine bay arrangements. If it all goes really wrong, you need to be able to steer and stop the car.