If you hosted your own SMTP server, it would be impractical for other people to use it, as it would only ever accept mail to local addresses to itself (where there might only be one), so other people just wouldn't bother.

Spammers are only interested in "open" mail servers - those which will accept mail destined for anywhere. As soon as your mail server rejects all non-local mail, people lose interest very quickly and go find some other mail server. This is the typical mail server configuration.