Because you are decelerating sharply
Inertia
'The car's inertia carried it forward several feet after the driver pushed the brakes.' 'His inertia launched him forward after reaching the base of the hill.'
If you are in a fast moving car and the brakes are applied suddenly and hard, your body will continue moving in the forward direction.
Inertia
The seat belt makes an balance forward force because when the car stops you fall forward and the seat belt makes it so the force in your body stays balanced. So it becomes an balanced forward force.
If all the passengers are properly buckled in and the car brakes suddenly to a stop, someone may spill their coffee.
When a car is moving forward and the driver suddenly applies the brakes to stop, the car's acceleration is against the direction of motion. This is because the acceleration due to braking acts in the opposite direction of the car's velocity, causing it to slow down.
Force of Inertia
It keeps you from moving forward if the car is hit or stops suddenly.
Weight transfer. When a car brakes, its weight moves forward and causes the front brakes to do the majority of the work in stopping the car.
When a driver suddenly applies the brakes, the car decelerates rapidly, but your body continues moving forward due to inertia, the tendency of objects to maintain their state of motion. This forward movement happens because your body is not directly attached to the vehicle’s braking system. As a result, you experience a jerk as your body lags behind the car's sudden stop. This phenomenon is a basic principle of physics that explains how forces affect motion.
The answer, in short, is INERTIA. Inertia is resistance to changes in motion, and is proportional to mass. Newton's 1st Law of Motion tells us that objects in motion will remain in motion (in a straight line) unless acted upon by an external force. In the example given, the passengers (objects) pitch forward when the driver hits the brakes in a forward-moving car (providing the external force) because their bodies possess inertia separate from the inertia of the moving car. The car slows down due to the braking, but the passengers' separate inertia(s) will cause them to keep moving forward until restrained by an outside force (seatbelt, dashboard, windshield, etc.) Larger, heavier passengers will pitch forward with more force (inertia is proportional to mass, remember?).