Sure, I'll give you some change for it. (Fumbles in pocket) Hmm... Comb Eyeglass case Pocket loupe Breath spray Lint Uh-oh. Seems I don't even have any spare change. Too bad. I would have loved to have taken you up on your generous offer.
There are several $10K notes on BEP souvenir cards. Fyi, if you see these in TPG slabs with the notation "intaglio print" or "proof" they are almost certainly cut down cards.
Short answer: To bilk unwitting buyers. I've seen these slabbed cutdown "notes" sell for hundreds on occasion, when the actual cards can be bought for $10 or less. Many collectors are unfamiliar with souvenir cards and think they are buying a proof of the original note. This was rampant about a decade ago and the Secret Service actually restricted the BEP from using complete currency designs on cards for many years. Here's a thread on another forum that details the practice.
Because some people will pay good money for any silly thing that looks like paper money encased in plastic festooned with a number in the high 60s.
Souvenir cards have been common in the philatelic market for years, and the cut ones are usually worthless. I'm well aware of this stuff. But the whole cards are lovely...why cut them...IDK