Here's why:

* Acceleration is the rate of change of velocity.

* Velocity has both speed and direction.

If a car is moving at a constant speed, its speed isn't changing. However, even if the speed is constant, the car could be changing direction, which would mean its velocity is changing, and therefore it would be accelerating.

Example:

* A car driving in a straight line at 60 mph is not accelerating because its speed and direction are constant.

* A car driving in a circle at 60 mph is accelerating because its direction is constantly changing, even though its speed is constant.