>>55900430They're particularly rare and powerful and don't breed in captivity
Koraidon/Miraidon in SV is a Legendary Pokémon even though it is explicitly just one specimen of a future species and you can even catch two of them, because it is still uniquely rare and powerful compared to the other Paradox Pokémon.
Cosmog and its family are Legendary Pokémon despite the fact that they can breed and evolve. Again, they are uniquely rare and powerful compared to the other Pokémon from Ultra Space, which are instead dubbed Ultra Beasts. There are other Ultra Beasts that the player can only obtain one of, but all of them are understood to be an abundant species on their home worlds, we're simply not given the opportunity to catch more.
A Pokémon is NOT Legendary just because it "has legends told about it." We hear "legends" about plenty of completely ordinary mundane Pokémon. Paradox Pokémon are explicitly treated as "legendary" monsters shrouded in myth and rumor from the Scarlet/Violet Book, but only Koraidon/Miraidon are actual Legendary Pokémon. There's no reason for this distinction to exist if it's based purely on reputation - if anything, we've made them much more mundane than the other Paradox Pokémon by showing them off around the whole region while the rest are locked away in Area Zero.
Conversely, there are a number of Legendary Pokémon who are explicitly unique or nearly unique and whose existence is known only by a select few individuals or groups, like Mewtwo and Sivally. It is hard to imagine how there could be "legends" about a Pokémon almost nobody knows about.
