Each AOSP version is just a big code drop from Google. It's really far away in the spirit. Not even talking about how difficult it is to patch some component as a user.
There's about 5% of commits made by people with e-mail domain different than google.com, android.com or chromium.org (6% if you count the last one too).
Do you get to see the code of yet unreleased versions of Android? Sure, they do accept some patches, but "developed in the open" doesn't seem to describe their development model.
Check out the current PRs in gerrit: https://android-review.googlesource.com/q/status:open+-is:wi...