Sorry im confused,but my question is

The velocity is not constant. The velocity will decrease as soon as the car hits the wall.

If we can find out the rate of acceleration of the car to stop then we can find the force exerted on the wall but since that is impossible to find and is usually not constant but keeps varying so in such situations we use the conservation of momentum princile to find the momentum change and then using time taken to stop and the second law we find the force exerted on the wall