I suspect that Google's reputation as "the best of the best" leads people who work there to think they're important, and important people don't waste their time

I suspect that Google's reputation as "the best of the best" leads people who work there to think they're important, and important people don't waste their time on trivial work like developing user sign up forms. To get this sort of thing right you actually need 'average' devs to take on the boring, every day things, and to care about how the code works for users more than shaving another tenth of a millisecond off of the FCP time.