Because I like collecting stuff. Not obsessed with it though. Probably why I can avoid the whole "OMG, there's like, a gajillion of them now with this new gen, I'll never catch'em all!" thing. Usually I just focus on the ones I like the most (and maybe put a little effort in getting their shiny version if I like the colors), and figure out how to make them an effective team. (the sword ghost and the new fire starter already have a spot.) I'll catch whatever ones I don't have as I'm going through the game, but I won't make an effort on getting every single one. I probably have 2/3 of the Pokemon throughout the series, and I'm cool with that.