You already identified the "usability" issues. It would need to be a system that required people to just get once a "code" for their computer. And then another

You already identified the "usability" issues. It would need to be a system that required people to just get once a "code" for their computer. And then another systems auto detect if the recipients has also a "coded" machine. Encryption is just inconvenient for the average user and doesn't provide some immediate benefit ("My bank needs encryption but I don't need to encrypt my funny cat pictures").