The problem is coming up with a scaling algorithm that works ideally on every kind of image; it simply can't be done. Some methods work better for some images, and other methods work better on others. Wikipedia has a good overview: http://en.wikipedia.org/wiki/Image_scaling

The scaling in the browser has to work on any arbitrary resizing, not just 2x. A simple pixel doubling only works in one specific circumstance, and would be inconsistent with other scaled images.