Because speed and velocity are not the same.
Velocity is defined by speed and direction. Speed is merely distance over time; cars go around corners, even on straight roads the bumps in the surface cause them to pitch up and down, and they must also constantly follow the curvature of the Earth.
Many things are said to have velocity such as bullets but even they obey gravity and so really have speed not velocity, but that's nit-picking for everyday purposes of description.
Because it is important for the driver to know their speed and it is easy for average person to understand what speed is.
Velocity is different. It is a combination of both speed and direction. Because a car is often changing direction this affects the average velocity. Most people would need to take a physics course to understand the concept and it not something the driver needs to know anyway.
You think a speedometer is NOT a variety of velocity meter? Why would you think that?
Velocity is "speed with a direction" but cars change direction all the time.