Having built a number of Repraps, “nearly everything” is highly exaggerated. I have seen 3D printers with an almost entirely printed frame, but using off the shelf T slot rails is a lot more time and cost effective.

It is currently not possible to print the control board, wiring, sensors, hot end, motors, heaters, bearings, slides and rails necessary for a 3D printer. Some of the mechanical parts and a lot of the bracketry that holds the frame together can be 3D printed.