I don't want to go too far down the conspiracy theorists' rabbit hole here, so just consider this some alternative reality fiction for a second: Because it is

I don't want to go too far down the conspiracy theorists' rabbit hole here, so just consider this some alternative reality fiction for a second: Because it is easier if everything is in one place. Imagine emailing back and forth with somebody, and they accidentally send you an email to the wrong account. It makes it easier to control everything.