1. Download, extract, and run the new version of Xcode.
Note: you cannot upload builds for production App Store with pre-GM builds. You _must_ use the GM or production Xcode.
It's a 12 GB download, and extracting takes 20 minutes. Factor in about an hour here. Especially around when it's posted you can expect a very slow download.
2. Build, execute and validate the app at least _launches_.
Maybe another 30 minutes.
3. Create and upload an archived build to Apple.
Maybe another 30 minutes.
4. Wait for the uploaded build to finish processing.
This takes somewhere between 1-3 hours.
5. Submit the finished-processing build.
Manual inspection of metadata, screenshot uploading, etc. Let's call it 20 minutes.
6. Wait for approval from Apple.
Who knows how long.
7. Release.
So this is somewhere around 4-5 hours at _minimum_ of work to get the build uploaded.
When you're told at 11am that users will have it in their hands the next day, even if you are incredibly prepared, there's simply very little time to actually do the above in a safe way. And if you're not on Pacific Time, you're going to miss this window entirely.
Not to mention Apple uploaded a GM build of Xcode around noon, and then silently replaced it with an updated version sometime later without explaining what the difference was. Fun stuff.
Also: the GM build has regressions.
Do certain vendors get pre-release access that smaller vendors do not? Does Twitter/Facebook/etc have to do the same thing you just described? If it was truly as bad as you are making it out to be, how in the world does the eco-system survive? There's no way Apple could expect to be able to handle reviewing every single app in this manner. Surely something is missing?
I also believe that, right now, they're just rubber stamping all app reviews. They cannot handle this volume.