There used to be a flag for that. Starting in 2006, there was a special "DPI Slider" in Mac OS X that allowed you to set the resolution from 1.0 to 3.0. Each WWDC they claimed that by next year they'd activate it as a user feature, but that never happened. Instead we got retina mode. The reason probably is that they never really got all apps and all use cases to look terrific in all supported scale variants. I remember playing with it a lot from 2007 to 2010 and usually there were always apps (oftentimes even Apple apps, like Textedit) that simply looked awful at 1.3 scale or other variations.

Indeed. Acorn's RiscOS had resolution independence in the 80s with scalable fonts and icons being shipped as vector graphics; it's a pity everyone else is 30 years behind.

I'm afraid you're mistaken. RiscOS certainly had scalable vector fonts (anti-aliased too!), but the icons and interface elements were all bitmaps and the OS was not resolution independent.