WHY does an acceleration cause velocity to increase

It doesn't always do that.

Acceleration means any change in the speed or direction of motion. So any time an object speeds up, slows down, or turns, acceleration is happening.

These 3 things are all changes of velocity. Acceleration causes them, because that's the definition of acceleration.

When the velocity of an object changes it is said to be accelerating. Acceleration is the rate of change of velocity with time ... Acceleration occurs anytime an object's speed increases or decreases, or it changes direction.