The, uh, game takes place takes place in the real world. The cards are the creation of extensive Nen restrictions. The whole reason why those effects are so powerful is the creator of the cards presumably can't use them and have no control over who gets them other than within the confines of the game's rules.
It makes no sense at all. Even if you want to take into account Nen restriction and what it can allow you to do, these cards are so varied and have such a wide range of abilities that they effectively being able to work in the rest of the world (I could understand with a lot of Nen manipulation why they would work on Greed Island itself... like you could argue it away by saying they infused the island itself with Nen to allow for that) is literally rewriting the laws of the world. It is lazy story telling so that he could just do a JRPG story since he was obsessed with Dragon Quest at the time.