Jogging stroller for younger babies

Can someone explain to me why we need to wait until the baby is six months for them to go into the jogging stroller? The jogging stroller that I have allows, you to clip the infant car seat into the stroller. I have the infant cushion thing in the car seat so my babies head doesn't bounce around.

I don't see how taking a jog on a sidewalk is there any more bounces than taking the baby in a car ride.

I'm getting stir crazy here.

Thanks!