Here's why:

* Coefficient of friction is a property of the materials in contact. It represents the ratio of the force needed to overcome friction to the normal force pressing the surfaces together. This property is largely determined by the surface roughness and the nature of the materials involved.

* Speed primarily influences the force of friction itself. A higher speed can lead to a greater force of friction, especially in situations involving fluid friction (like air resistance or water resistance). However, the coefficient of friction remains relatively constant.

However, there are some caveats:

* High speeds can lead to changes in the surface. Extreme speed can cause wear and tear on the surfaces, potentially altering the coefficient of friction. For example, a tire spinning at a very high speed might heat up, leading to a decrease in its coefficient of friction.

* Certain types of friction are speed-dependent. For example, viscous friction (like the friction experienced by an object moving through a fluid) is directly proportional to the speed of the object.

In summary: While speed influences the force of friction, it generally does not significantly impact the coefficient of friction, which is a material property. However, there are specific situations where speed can have an effect, like in extreme conditions or with specific types of friction.