You're perhaps under the impression that OpenSSH was the original implementation: it's not, it's a relative latecomer :) The original SSH was (and still is) commercial software, see ssh.com. You basically end up with three groups:
- The original SSH and it's commercial competitors. Some abandoned, some still surviving.
- Various open source implementations. OpenSSH and PuTTY were the winners (PuTTY actually was released first). But there were more attempts (e.g. lsh).
- Those early open source implementations were designed as stand-alone programs (clients and servers). So you have a later wave of library implementations. With of course at least one per language.