I would imagine that it is a fabrication problem. I'm not sure but I would think that steel tubing atleast would be extruded into the shape of a tube. I would think making dimples would be a process that would take further work on the tube once it has been extruded. If the tubing is made by rolling sheets of metal the dimple process might be easier but then they have to weld the two sides of the tubing together for the length of the tube (ie. more work).