It many be due to costs or that the brake discs would have to be made out of aluminium/copper for the electromagnetic braking to be effective or there may not be a significant reduction in braking distance. I'm just not sure if these are valid reasons or not.
The existing brake systems consisting of hydraulic pumps and calipers and disks have proved to be, reliable, efficient, low maintenance, and fail-safe.
They are powerful enough that from the moment of deployment they impart maximum practical deceleration right up to the level of the limit stopping force short of skidding. There is even a need for a governor control system ( ABS) to rain in on their performance not to allow the potential lock-up of the wheels.
The possible bottleneck is in the tire's friction and heat resistance capabilities.
Adding any other system will multiply the complexity, increase the chance of failure, and add to the cost of the car and its maintenance.