How so? These are reference cards, so frequent use would be expected. I would also imagine that a great many of them would be carried around by their owners to review in their spare time. (I certainly did so in my younger days.) Their owners would expect them to be durable, to avoid things like the worn and splitting edges that are ever so common on paper products that see a lot of handling.
Keep in mind that these are things that people would set out to buy on their own. Plastic, or at least durability, is something the consumer would look for. They aren't something thrown into the box with an IC, where the vendor would do everything they could to reduce the cost.
Keep in mind that these are things that people would set out to buy on their own. Plastic, or at least durability, is something the consumer would look for. They aren't something thrown into the box with an IC, where the vendor would do everything they could to reduce the cost.