Do you think there's a difference between "can run" and "runs stable and performing"?
Semantically, sure, but the AOSP builds for the Nexus phones are stable and performant. The differences between AOSP and the official Google firmware images are mainly skin-deep, in the form of UI themes and preinstalled apps. In other words, things which do not require a great deal of work to port to each new version of Android.
Are you trying to tell me the Nexus system image is exactly the same as AOSP?
Google has their own customizations and add-ons just like the other manufacturers. You can build and install fully functional AOSP images on Nexus devices, but they don't ship with stock AOSP. However, Google's changes are layered on top of the system using the AOSP mechanisms designed for that purpose, which reduces the porting effort considerably compared to other phones which need proprietary binary drivers (and thus specific kernel versions) and deep modifications to core AOSP components.