It takes even more time to read and understand a patch. Not only to you have to do all of the work of reading and understanding the bug report for which the patch is relevant, but you now also have to read and understand the submitted code, which until just this moment you didn't even know was necessary and have no specific context for. Then you have to validate whether or not the code in the patch is sufficient to fix the issue or whether those changes have any additional knock on effects. Yes, you could hope that the Google coders did this, but since you already have such a low opinion of Google's efforts and behavior on this front, I would argue that trusting their submission without validation would be insane.

Then if there's any changes or additional work to be done, you now have to spend time communicating with the patch sumbmitter, either getting them to make the requested changes, or rejecting their patch outright and writing it on your own.

And after all that we'd be right back here, only instead of the complain being "we don't have enough time to review all your bug reports" it would be "we don't have enough time to review all your PRs"