I was wondering the other day why don't all three motors move at the same time? Don't normal paper printers move 2 motors at a time? they're 2D printers. It makes sense if a 3D printer really does print with all three motors moving. Won't it also be more efficient if they do 3D print in all axes?
I was wondering the other day why don't all three motors move at the same time?
That is perfectly possible for most printers (with limitations, "3D Printing" with all steppers being used is called "non-planar" printing), but there are some major cons you need to deal with. First, there are not that many software suites that slice objects perfectly well like the "normal" per "fixed/variable layer height" slicing as the slicers are under developed. Second, it requires a specific tall and slender nozzle/print head, else the height of printing is very limited. Last, besides generally not being useful for rectangular prints (except for 90° overhanging structures1)), this method is only/best suitable for curved objects as the top layer will follow the contours of the object.
It makes sense if a 3D printer really does print with all three motors moving.
No, it does not make sense, the essentials of 3D printing it that it produces a 3D object, not that all stepper motors should be running simultaneously.
Won't it also be more efficient if they do 3D print in all axes?
Not necessarily, there are limitations to non-planar printing like geometry and print quality, but, you could (in some cases) print with less support material.
Note that these are demonstration pieces, normally you would rotate the print for 90° and print it sliced normal (planar).
Frame challenge: is it just 1D printing? Each G1 command moves linearly in a single one dimensional affine subspace of the build volume.
What makes 3D printing 3D is that the resulting object produced is three dimensional. There are indeed a lot of advances to be made in FDM printing by not working only within constant-Z cross sections at a time, but these are matters of surface quality/accuracy and the classes of geometry that are printable, not efficiency.