Why is it when someone has a pipe rack like a 4" pipe?

Question

why don't the pipes slide out from inside of that pipe?????

Answer 1

If you ever notice a pipe truck usually have boards in between each layer of pipe and at the end of the boards on both sides is chocks this keeps the load from rolling around then you strap it down (3 or four straps) and away you go.

Answer 2

They are likely strapped down and secured...