The compiled kernel isn't distributed so there is no need to distribute source. Note that prodkernel is different than the Android kernel (where they do, and are required to share the source by GPL).

To be honest I don't think it is a major loss. IIUC most features are either only interesting internally, or are submitted upstream to reduce maintenance cost. Google's "secret sauce" isn't how great their kernel is, so I am not aware of anything that is held back for competitive advantage other than maybe some drivers for custom hardware. So while it would be nice to see all of these patches collected into a "live" snapshot of their kernel it probably wouldn't be that much more helpful (if at all) to a third-party.