Credit card numbers are a frequent target of hackers. And it's no one's business but yours. You may be planning a political campaign, discussing our 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. If you really are a law-abiding citizen with nothing to hide. 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. Fortunately, we don't live in that kind of world, because everyone protects most of their mail with envelopes.