A lot of people get a credit card and leave it in the envelope for a considerable amount of time.
Further, separating the data complicates life very greatly for a mail thief. To snatch one piece of mail is a crime of opportunity. But to snatch two on separate days requires veritable stalking. Having gotten one piece, the thief must now return to the scene of the crime often. That greatly increases exposure, both because of the lingering and the repeated visits.
Further, it's likely the correspondence is not obviously marked with a sender, so the thief does not know which piece of mail to steal, and must steal a lot of it. This greatly increases the chance of the owner noticing their mail is going missing, which would defeat the entire exercise as the owner would cancel the card.
And after all that, there's a fair chance that the piece the thief lifted was the second to arrive... In which case the other half of the puzzle will never show up. The thief could check for weeks and never get it. When to give up?