An issue is for most sites/services an email has just become a standard authentication method, rather than something that can easily be more unique per account. So any usernames across sites/services that share it identify that user as being the same person (for data broker profiling, doxxing, etc), which is the privacy issue (not the email address per se, unless it perhaps contained one's real name).
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.
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.