In Apple's defense, they've been working on full resolution independence since 2005 or so, with a setting for developers available from Leopard (2007) on, where one could set the resolution in between 1x to 3x. However, due to the complexity of the issue, Carbon apps, and probably a lot of other things, that never worked really well. I've been playing around with it with each new OSX release, until they removed it (I think Lion or Mountain Lion).

Basically, I think they never got it working good enough and just fixed it to 1x and 2x.

Here's a screenshot from the utility they had for this. http://i.stack.imgur.com/0NToe.png