At the risk of being serious, I think there are several reasons. One is indeed that amps are too big, heavy, or heat-producing to go in a rack - I've used a low (4" or so) amp stand myself. A second is that it may be convenient to put the gear that requires interaction (preamp, TT, CDP) in a rack some distance from the speakers, while the amp is placed to minimize speaker cable length. A third is that gear between the speakers on a higher rack may be perceived as a source of diffraction, compromising the imaging. And, of course, there are always the dedicated followers of fashion who've seen such setups elsewhere ...