You can only print parts of a 3D printer on a 3D printer, and one of the main parts is the printer's frame.Printers have many metal parts that need to be highly accurate (example - metal rods on which the extruder and bed slide to move in different directions) to get a high-quality print, even if the parts can be printed with a metal 3D printer, achieving the required surface quality will be difficult and expensive.
Some parts of the extruder can be 3D printed (like the gears) but not the hot end - because as its name implies, it reaches a high temperature at which many plastics melt.
Then there are the electronic components - motors, servos, etc. These can't be 3D printed. Circuit boards can be printed but the technology hasn't advanced enough to reduce the cost of the board when compared to one being produced by conventional methods.
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