I'm going to pull a 640k-ought-to-be-enough-for-everybody, but hear me out:
720p 4xAA is pretty much sufficient for mind-blowing visuals. Why stress the hardware unnecessarily?
Would you be bothered if your game looked as good as a high def (720p) movie? Would you be bothered if your game didn't run at 1080p, but with real-time caustics, global illumination, radiosity, sub surface scattering, motion blur, realistic DOF, sub-pixel displacement mapping, fur / hair rendering, voxel particles, skeletal / muscle simulation, gas / liquid simulation at 720p 60fps?
As if we got all of these running together at 720p, we want our visuals to run at 1080p..
So why the 1080p madness? The games don't look all that different at 1080p from 720p.. It's the rendering, stupid..
This looks more realistic:
Although the first picture effectively has quarter the amount of pixels of the second, and upscaled with the simplest scaling algorithm, not even with the fancy methods most quarter-decent tv sets use, it looks more realistic than the second picture. Mind you that pixel count difference between 720p and 1080p is not even this dramatic.