Perhaps for the same reason that speaker crossover components get sealed and sometimes potted in their own sub chambers or circuit boards and transformers suspended on squishy interfaces or tubes inserted in special gel-filled sockets?
I know reputable manufacturers who have taken the same circuit (transistor preamp for example) and encased it in different chassis, then experimented with different ways to minimize resonant behavior and such. Bottom line? Very audible differences with no change to circuit board layout, parts or anything - just what the enclosure material was, how it was shaped and how different bits and bobs were isolated.
Eduardo de Lima of Audiopax stuck his amp circuit inside a brass case, aluminum case and stainless steel case. No other difference. The brass case won by quite a margin. He couldn't explain it but made the choice his ears told him.
Resonance control in audio can be very beneficial. Not all of it seems to make sense on the face of it and not all of it makes for better, just different. But it's easy enough to experiment for yourself and draw your own conclusions. If you're performance driven, you may not need an explanation to embrace a solution that to your ears seems to work
