And to add to it: macOS (and its derivatives, such as iOS) descends from an operating system called OPENSTEP (formerly NeXTSTEP), which used Display Postscript for on-screen rendering.
Apple later replaced Display PostScript with a more modern reimplementation called Quartz 2D[1], which switched from Postscript to PDF.
Why not PNG? Because PNG is a bitmap format and Quartz 2D is designed to be resolution-independent.
Why not SVG? Probably because Quartz 2D relies heavily on the PDF specification throughout and it would be too much work to reimplement, and for what? PDF is still a common and well-regarded format.
its a more efficient vector format than PNG or SVG.
This is the correct answer.
And to add to it: macOS (and its derivatives, such as iOS) descends from an operating system called OPENSTEP (formerly NeXTSTEP), which used Display Postscript for on-screen rendering.
Apple later replaced Display PostScript with a more modern reimplementation called Quartz 2D[1], which switched from Postscript to PDF.
Why not PNG? Because PNG is a bitmap format and Quartz 2D is designed to be resolution-independent.
Why not SVG? Probably because Quartz 2D relies heavily on the PDF specification throughout and it would be too much work to reimplement, and for what? PDF is still a common and well-regarded format.
1. https://en.wikipedia.org/wiki/Quartz_2D