Because the MSAA still isn't fixed from BF3 and leaves jaggies on half the things on screen. Using the resolution scale to supersample is horribly inefficient as well and not everyone can afford to SLI Titans. Post AA to Low or Medium (High is way too blurry) is a decent compromise IMO.
That's what I do. Post AA to low MSAA to 4x or even 2x and everything looks pretty damn good.