This is a great question, but the answer is pretty mundane. Building out, operating, and maintaining the kind of tooling and infrastructure you need to make this work is really freaking hard even with good client-side support for digitally signed email. GPG and similar tools are really hard to use, really hard to support, and really hard to integrate with platforms and services. Even organizations that have rolled out smartcards to all their users and live entirely in a Microsoft world tend not to use digital signatures. Sometimes, integrations break, which results in the email client (or some other tool) breaking, and then the business stops while the integration gets fixed, if that's even possible. And Goddess help you if you're trying to support more than just Outlook on PCs, which has relatively good S/MIME support, all things considered. It's really, really non-trivial to do what you're asking.