Why do the brakes get harder and harder when the car is off?

Because today's brakes have a vacuum booster. This means that the harder you press on the brakes, the harder they get because that's how hard the brakes really need to be.

The booster relies on the vacuum created when the engine is running to create the boost, and when the engine is off, the vacuum is gone. The vacuum stored in the booster itself gets less and less, so it naturally gets harder and harder.

As shown in the figure for a car's vacuum-assisted (direct-acting) servo braking system circuit diagram, which uses the left front wheel brake cylinder and the right rear wheel brake cylinder for a hydraulic circuit, the right front wheel brake cylinder and the left rear wheel brake cylinder for the other hydraulic circuit arrangement, that is, for the diagonal arrangement of the dual circuit hydraulic braking system. Vacuum booster air chamber and control valve combination of vacuum booster in the work of generating thrust, but also the same as the pedal force directly on the piston of the brake master cylinder pusher.