Here's why:

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

* Velocity is a vector quantity, meaning it has both size (magnitude) and direction. It tells you how fast something is moving and in what 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.

Therefore, speed is not fully described by magnitude and direction. It's only described by magnitude.