Here's why:

* Acceleration is the rate of change of velocity. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Constant speed implies the magnitude of velocity remains the same. But, for acceleration to occur, there needs to be a change in either speed or direction.

* If the speed is constant and the direction is not changing, the car is not accelerating. It is simply moving at a constant speed.

Example:

Imagine a car driving on a straight road at 60 mph. If it maintains that speed without changing direction, it is not accelerating. Even though it's in motion, its velocity isn't changing.

However, a car can have constant speed but still be accelerating in some situations:

* Circular motion: A car driving in a circle at a constant speed is still accelerating because its direction is constantly changing. This type of acceleration is called centripetal acceleration.

In short, acceleration requires a change in velocity. Constant speed means no change in velocity, and therefore, no acceleration.