For 3D printing to work, you basically just need some standard motors to move a thing that gets hot around. Yes it needs to be pretty precise, but it's only printing at a single point that moves. Classic 2D printing not only prints across the whole sheet at once, it is also sometimes expected to do it in color (which does take multiple passes usually). As for the technological aspects of conventional printers, I really don't consider myself an expert, there are great videos online. However by my understanding, a laser is often used to trace the exact contents of the page so that the depositing material is picked up and placed. That sort of light manipulation is already more complicated than everything most 3D printers do.

Basically, you could build a new 3D printer in your garage using off the shelf parts and some knowhow, but good luck even repairing a 2D printer with a serious problem in its printing mechanism, though this difficulty is certainly not made better by companies such as HP