Hey, just like there's a market for things magic and cardistry, there's a market for other cards-related ephemera, and those collectors will take used decks. It's rare to find older decks that are still sealed, and they're generally an order of magnitude more expensive.
The single-swappers like a certain aspect of card design, like the Ace of Spades, the Joker, the card back, etc. and will collect just that card from the deck - it's a less expensive way to get into an expensive hobby.
Just filter out the stuff you don't want like Loldudex2 does and you don't have to see what you don't want to. Personally, I can get lost just window-shopping some of the assorted vintage playing cards for sale, not always just the new, limited-edition, magic-oriented, etc.