Infinite theoretical printing supply. In reality, that specific really cool Charizard from my childhood stopped being printed decades ago, and natural wear and tear reduces the supply compared to 1999 (or whenever it was printed). Nostalgia kicks in, demand skyrockets. I don't know why Pokemon specifically did this, but it's far from the first trading card series to have this effect.

As for utility: it's still entertainment. remember that this is a TCG, so there technically is still a formatted game to play with these cards. Even if most effects in older cards have been long irrelevant in competitive formats, other TCG's can have card prices surge due to new strategies being discovered.