> Why then do many servers require their websites to communicate through the www subdomain?

You mean host. www is a host in example.com. Protocol indicator (http/https) and host name (www, mail, bob) have nothing to do with each other.

You don't have to do @host.example.com if there is a special DNS record for SMTP to indicate where to actually send mail, without it you do have to do host.example.com.

> automatically connect to the HTTP server on port 80

What HTTP server? if the URL is www.example.com we connect to the server named www, it could be named bob if we wanted. Without the host name given you have to create a catchall to tell clients to send ALL traffic without a hostname to.

The comparison between protocols is really stupid.