That is circular reasoning. Bitcoin is also a collectable, with a fixed number of them (21 million) out there to collect.
Every investment we are talking about is only rare because the community that participates in it has decided they are rare. Otherwise there is nothing stopping me from taking a picture of a piece of art or trading card and reproducing it a million times.
Gold is also fungible, as is any physical or virtual currency, water, oil, any sneaker run, a collectible figure from a video game. Non-fungibility is not a requirement for value.
Every investment we are talking about is only rare because the community that participates in it has decided they are rare. Otherwise there is nothing stopping me from taking a picture of a piece of art or trading card and reproducing it a million times.