OSX is unfortunately bitmap based
What is that supposed to mean? A lot of artwork comes in bitmap format. But there are also quite a few PDFs and the OS doesn't really care either way.
The graphics system is point based. Where a standard display features one pixel per point and a high dpi display has two (by two) pixels per point.
Apple tried to make the UI scale arbitrarily, (the feature was available for development purposes for years) but it didn't really work all that great, because there are too many cases where you get off-by-one errors that look quite bad. They eventually decided that it wasn't worth the effort and instead opted simply for displays with a 'high enough' resolution (i.e 'retina' displays) and integer scale factors.