It has nothing to do with the slogan. In fact, it stopped being the game’s slogan by gen 3 (look up Crystal’s box art and then look up any game’s box art after that like Sapphire). Catching them all is just a fun side objective completionists can do, similar to collecting all the achievements in a game. I’ve also collected all of the creatures in other creature collectors like cassette beasts or monster sanctuary because I find it fun, not because some 25 year old slogan told me to.