Default [Q]A question about AOSP

We know that there are lots of AOSP based ROMs, such as CM, omni, etc. But why nearly all the developers don't like to migrate original AOSP to our phones, but only CM, omni, carbon, and so many custom ROMs? What's wrong with the original Google code? Anyone know the reason about that? I know that these custom ROMs have many additional features, but I just like pure Android.