Circles do not exist

However almost every circle" you can see in printed media (and most purely digital ones) are not, in fact, circles. Why is this?

Since roughly the mid 80s all high quality" print jobs have been done either in PostScript or, nowadays almost exclusively, in PDF. They use the same basic drawing model, which does not have a primitive for circles (or circle arcs). The only primitives they have are straight line segments, rectangles and Bezier curves. None of these can be used to express a circle accurately. You can only do an approximation of a circle but it is always slightly eccentric. The only way to create a proper circle is to have a raster image like the one above.

Shouldn't be that big of a deal, right? I'm sure nobody is using PDF for anything that would require the kind of precision needed for a perfect circle, like CAD drawings for laser cutters and similar machinery. Right?

Again one might ask whether this has any practical impact. For this case, again, probably not. But did you know that one of the cases PDF is being considered (and, based on Internet rumors, is already being used) is as an interchange format for CAD drawings? Now it suddenly starts mattering. If you have any component where getting a really accurate circle shape is vital (like pistons and their holes), suddenly all your components are slightly misshaped. Which would not be fun.

This is why we can't have nice things.