Because you are decelerating sharply
'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.'
When a car stops suddenly, the passengers tend to move forward relative to their seats due to inertia. Inertia is the tendency of an object to resist changes in its motion, so when the car stops suddenly, the passengers continue to move forward at the car's original speed until the seatbelt or seat stops them.
When a driver applies the brakes quickly, inertia causes the book to continue moving forward momentarily before friction and the force of deceleration from the car's brakes eventually bring it to a stop. The sudden stoppage of the car transfers inertia to the book, causing it to move forward until external forces act upon it.
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
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.
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.
Inertia, which is the tendency of an object to resist changes in its motion. When a car stops suddenly, your body continues to move forward due to its inertia until an external force, such as the seatbelt or airbag, stops it.
It keeps you from moving forward if the car is hit or stops suddenly.
Force of Inertia