No mention of the reason why. Seems like a foundation feature. Ran into trouble more than once by their bad auth system.
My five cents for a guess: they've used the email as an index somewhere, which massive amount of data now depends on and they keep pushing forward the need of migrating away from that because it'll be painful and take long time.
I’d say it’s a rookie mistake, but they’re a half-trillion dollar company, so maybe I _should_ use email address as an id?