The worst thing is that they don't to do that. Some of the wackier models/animations in the GameCube Pokemon games show that there's literally no reason whatsoever their default poses can't be more dynamic, except for... y'know, laziness. So, really, the same reason for most things wrong with more recent Pokemon games. Edit: Okay, let me amend that to "inefficient use of disproportionately large resources." Though realistically, Pokemon's irrationally short turnarounds between games are the cause for some of the substandard elements of recent titles. Sort of ironic to accuse a dev of being lazy because I myself was too lazy to properly word my criticism.