.stl files most commonly used for printing do not have any circles in them at all!
.stl files describe the surface of the things they represent with lots and lots of triangles:
That also means that your slicer doesn't really know which parts of the model are supposed to be round (where
G3 might be applicable).
And, another consideration, though historical: Most 3D printers were (and are) based on relatively weak 8-bit microcontrollers. It's much easier for those microcontrollers to calculate the movements needed for the print if it's just straight lines, rather than elaborate curves.
One issue that can arise with this is when your model is of high quality, and your slicers uses all of that quality - a simple circle can then become hundreds or thousands of tiny segments, which all need to be transmitted, parsed and executed - which may lead to stuttering.