There's a possibility that he lost out in internal political battles, and things weren't going his way. Google is full of battle-hardened political warriors who will do anything (subterfuge, sabotage, etc.) to win battles. It is possible that a guy who just wants to build cool shit would feel like a misfit in such an environment.
Noam Shazeer is not some super brain anymore.
His raw skills are likely atrophied by now from delegating and operating at higher levels. His entire value is mostly political now.
If he can’t play the political game and win, he’s useless. He would probably just go to some other organization where people aren’t immune to his lore.
But he worked there previously and is Intelligent so knew all that before rejoining?
True. But earlier he had the ear of people like @jeff ; but now even Jeff has been sidelined a little (Google Brain is no longer under Jeff, AFAIK). The MBA types can be brutal and since they're not technical, they don't have the same level of respect and deference for Noam.
Google is a different place today than even 5 years ago.
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