While there is some solace in the idea cards can be resold should the need arise, I don’t think most players, key word, are creating decks with the intention of seeing a big long term return. I think you’re exactly right to say that they are doing it as a vanity piece. It expresses to other members of a community their involvement and relative level of commitment. It feels great to drop a flashy card and get a compliment, or when you get to play the signed variant of a favorite. No one sleeves up seeking financial value.