Although care has been taken to shield the rest of the components within the chassis from interference, there will always be some influence, however small. And, as Gregm mentioned above, there can be space issues... Bigger, better, more robust PS's need more room... but not only that, take it further, and factor in heat issues. With electronics so closely huddled together, having an outboard PS, eliminates the need for added bulk to shield the heat. The thinking is that even with shielding, run the component long enough, and it will get hot.
H
H