Here's why:

* Speed is a scalar quantity, meaning it only has magnitude (how fast something is moving).

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

Example:

* A car traveling at 60 mph has a speed of 60 mph.

* A car traveling at 60 mph east has a velocity of 60 mph east.

So, to be precise, we don't talk about "speed in a given direction". We use the term velocity to describe the combination of speed and direction.