Here's why:

* Velocity is a vector quantity: It has both magnitude (speed) and direction.

* Speed in UCM is constant: The object moves at a constant speed around the circle.

* Direction is constantly changing: As the object moves in a circle, its direction of motion is always changing.

Therefore, even though the speed is constant, the changing direction means the velocity is not constant.

Instead of velocity being constant, we talk about the object having a constant *angular velocity*. This means the object covers the same angle in the same amount of time, even if its linear velocity is changing.