Although I've seen a variation or two on cabinets, maybe with a reflex port and the odd mixed speaker arrangement, why is it not pretty standard to have a mixed speaker cabinet?

I think a stack generally consists of 8 x 12" speakers. Why would there not be some 12,s & 10,s for bass frequencies, 8's for midrange, and horns or something for high frequency sounds?

As compared to stereo speakers, we need a range of speakers to get full frequency playback. Why is this not done with guitar systems?