The force of gravity points straight down, but a ball rolling down a ramp doesn't go straight down, it follows the ramp. Therefore, only the component of the gravitational force which points along the direction of the ball's motion can accelerate the ball. The other component pushes the ball into the ramp, and the ramp pushes back, so there is no acceleration of the ball into the ramp. If the ramp is horizontal, then the ball does not accelerate, as gravity pushes the ball into the ramp and not along the surface of the ramp. If the ramp is vertical, the ball just drops with acceleration due to gravity. These arguments are changed a bit by the fact that the ball is rolling and not sliding, but that only affects the magnitude of the acceleration but not the fact that it increases with ramp steepness.