For contrast truly unique email aliases for example aren't possible on common services like free Gmail*, only things like self-hosting/certain paid email hosts, which makes less feasible for many. So from a privacy perspective while in an ideal world everyone would be able to freely create entirely unique per-account creds we're mostly stuck with the email implementation.
* One could create entirely separate accounts but it's high friction and IIRC the same phone number (now a requirement) can only be used for 2-3 accounts.