As far as my girlfriend is concerned: Because as soon as a piece is remotely nice its pockets are decorative jokes. If there were clothes that were both stylish and had decent pockets she'd buy them at once, but there are almost none. There seems to be no good reason for that to be so, extending the joke pockets usually doesn't cause the piece to look bad or lose function (if not stuffed to the brink and excluding those items where that can't realistically work).