Here's why:

* Velocity is the slope of a *position* vs *time* graph. The change in position over time is velocity.

* Acceleration is the slope of a *velocity* vs *time* graph. The change in velocity over time is acceleration.

Think of it this way:

* Position: Where an object is located.

* Velocity: How fast the object is moving and in what direction.

* Acceleration: How quickly the object's velocity is changing.

Therefore, to find velocity from a graph, you need to look at the position vs time graph, not the acceleration vs time graph.