App developers cannot write code that correctly renders at an arbitrary multiplier. It's been tried and it never works. Having 1x mode and 2x mode does work, which is why Apple uses that approach.
The real answer is that there is an industry of designers accustomed to doing per-pixel tuning of their art assets and this work is worthless if the assets are going to be scaled anyway. Selecting integer multipliers (so far: "2") allows those assets to continue to render as designed on retina displays.
Surely there are aesthetic reasons to prefer this in many cases, but it's not nearly as cut and dry a point as some people tend to think. Most recent Linux desktop themes are using vector assets, for example, with quite good results.
Whether the Mac/iOS world is a community of pixel-exact perfectionist artistes or a bunch of dinosaurs who refuse to use modern tools is a flame war for a different thread.
The real answer is that there is an industry of designers
accustomed to doing per-pixel tuning of their art assets and
this work is worthless if the assets are going to be scaled
anyway.
Look at this Folklore.org story on the origin of the roundrect in System 1: http://www.folklore.org/StoryView.py?story=Round_Rects_Are_E... It was only through a combination of Jobs' perfectionism and Atkinson's ingenuity that such a simple thing as rounded rectangles could be accomplished on the original Macintosh; can you imagine how long it would have taken to render a Finder window full of vector icons?
Look at Display Postscript, which was created by NeXT in the late 80s and carries through to much of Apple's Quartz rendering system today. Why are we still limiting ourselves to pixel graphics?
While, I'd probably prefer to have vector graphics for desktop assets just for the flexibility, hand-tuned scaled images usually look better than their vector scaled siblings.
Maybe I'm misunderstanding what you're saying. In case I'm not, here are some counterexamples.
Check out the first two pictures in this thread (most things on that UI are clickable):
https://forums.station.sony.com/ps2/index.php?threads/leaked...
Clickable chat UI (Planetside 2):
http://img138.imageshack.us/img138/6746/planetside2201302131...
Clickable UI within a 3D world (Doom 3 had a bunch of these):
http://static.gamespot.com/uploads/scale_super/gamespot/imag...