Why wouldn't someone follow the standard of prefixing it with an appropriate subdomain? E.g., `smtp` or `imap` if it's something they're going to be discussing

Why wouldn't someone follow the standard of prefixing it with an appropriate subdomain? E.g., `smtp` or `imap` if it's something they're going to be discussing publicly? Furthermore, why not use port numbers as a suffix? That's how we can have umpteen services across different protocols on a single domain.