Like everyone else, they just don't see the need to hide their own email 'cause they're not saying anything they'd want hidden. And it's no one's business but yours. You may be planning a political campaign, discussing your taxes, or having an illicit affair. Or you may be doing something that you feel shouldn't be illegal, but is. There's nothing wrong with asserting your privacy.
Privacy is as apple-pie as the Constitution.
Perhaps you think your E-mail is legitimate enough that encryption is unwarranted. You must be a subversive or a drug dealer if you hide your mail inside envelopes. Or maybe a paranoid nut. If some brave soul tried to assert his privacy by using an envelope for his mail, it would draw suspicion. Perhaps the authorities would open his mail to see what he's hiding.