Phonenumber is a weird argument. We discussed it quite extensively with a recent application that utilizes phone as the user but that's because it is a pure mobile item. We added email as optional years ago and with a recent subscription and privacy concerns, enforcing a secondary fall back to email as a form of confirmation on device changes because of things like sim swops and fraud. One of the arguments is that EVERYONE has a email account who owns a smart phone as you need one to actually use the phone properly, the software doesn't run on feature phones.
Not the same as this case but yeah, bit of trivia and probably where the mentality comes in that it was developed in a mobile first location and hence cell number as main user "descriptor".