I expect the main answer is that the code was written before
FILTER_VALIDATE_EMAIL existed, and no-one has felt the need to change it.
The PHP documentation for
FILTER_VALIDATE_EMAIL says "In general, this validates e-mail addresses against the syntax in RFC 822, with the exceptions that comments and whitespace folding and dotless domain names are not supported." This could be a problem if we wanted to allow people to use, for example,
name@servername as their email address.
If you believe that this is problem that needs to be fixed (it would be helpful you can give a specific case where it causes problems with another system), then you can raise an issue on the tracker.