The lumen of a Groshong is around 9F where as a dialysis catheter is around 13.5 - 14F. The Groshong would not support the required large volume of blood that is needed for dialysis.
As for the length, they both reach into the superior vena cava and into the right atrium, so lengths are about the same.
The groshong has a 2 way valve on the catheter tip that prevents backflow into the catheter, this valve would impede the needed flow for dialysis.
As for not needing to flush the catheters, the dialysis catheter can use Tego Caps that when flushed with normal saline, provides a positive pressure at the tip to prevent backflow and no heparin dwell is needed.