My reason is that I send mail from lots of different places like my iPhone, Apple Mail on the desktop, Thunderbird on a Windows box, and the GMail web client.
So synchronizing all of those with the right keys and signing software would be either a pain or impossible (i.e. iPhone and GMail on the web).
And on top of it, I doubt more than 1% of the people I email would even know what the signatures were, or would know something was up if an unsigned message came through. Or would even bother to verify the signatures even if they did know what it was.
Everyone you're mailing also has to have software installed to do the verification. I've gotten signed emails before, and do I check to see if it's legit? No. What would it even matter? Most of what I send over email isn't anything security-worthy anyways.
