If you have some insider information I'd love to hear it, because this sounds totally implausible:
"I work at the world's most powerful tech company, get paid embarrassing sums of money to do it, and get good reviews from my manager. But I don't feel super smart, so I quit."
So, two things: Google is weirdly self organizing, which can take some serious getting used to. Your manager isn't there to tell you what to do, YOU are supposed to figure out what you should be doing for the most part, aside from some broad strokes from your tech lead (who isn't your manager). This really messes with a lot of peoples internal hierarchy compass, which leads to feelings of being lost and directionless.
When you come in as a newbie, and you see all these people seemingly directing themselves effortlessly, and creating cool shit day in and day out, it can be seriously soul crushing. You're still on your tricycle trying to figure out how to do anything at Google, and aside from your laptop, virtually everything is a technology that is Google specific and shares almost nothing with anything you've touched before.
Between those two things, a lot of people feel like they'll never make it. They hold on for their 1 year anniversary if they can and then shop their new Google experience around to find a good exit.
"I work at the world's most powerful tech company, get paid embarrassing sums of money to do it, and get good reviews from my manager. But I don't feel super smart, so I quit."