In the last few of years Platinum and Diamond status have added expiration dates which expire in March of each year.

You must maintain earning 3,000 reward credits to stay a Platinum and 10,000 for a Diamond.

IMHO for cards not having an expiration are the following...
The card was issued before they put on expiration dates.
The guest was at gold level and a platinum or diamond card was accidently printed. (Wrong cards in the printer or computer terminal accidently connected (software default) to the wrong printer.