> they are not the ideal identifier for products
They are a pretty decent identifier for users. Facebook demanded my phone number for "security" purposes then turned around and used it as a unique identifier to tie me to purchase records from various businesses (e.g. Ticketmaster - why am I not surprised?) that apparently have shady tracking/advertising deals with Facebook.
This sort of information misuse and tracking (not to mention spam) is precisely what "sign in with Apple" was supposed to fix in terms of e-mail addresses, but it's currently instantly neutralized by SMS "verification" since it's a huge pain to get a different phone number just to prevent Facebook from using it as a tracking identifier.
(I guess this reminds me why I have said "no" to multiple inquiries from Facebook. The thought of being told to implement a dark pattern as part of your job is extremely unpleasant. Facebook's confusing anti-privacy settings are another example.)
And it's not just Facebook. This is seeping into other completely unexpected applications; for example I bought a game controller whose driver software setup demanded a mobile number for "verification" purposes. No, just no.