I design power amps.
Simple: cost. The added chassis adds a LOT of cost that could be invested in more important improvements. And it hits again raising shipping costs, also a LOT. The advantages can be minimized through shielding of the only noisy parts - the transformers. Finally, all power amp signals are fairly large, so they are not terribly suceptible to noise compared to things like DACs and RIAA (phono) stages. The input to a power amp is, by convention ~ 1V rmp for full output. The input of some moving coil cartridges, by way of comparison, is 1/2 of 1/1000th V (in other words 00.5 mV). Line stage preamps fall int he middle.
That said, for a recent prototype that i constructed i did just that - mostly to make it easier to work on and experiment with tho.....but everyone seems to thinkl its cool. :-)