Could it be that, non musician, collectors like instruments to display on walls. An amp wouldn't have the same 'wow' factor to them.
e.g. A big black box with knobs on isn't as exciting to them as a Gilmour Strat.
Years ago I picked up a Trace Elliot MP11 that had, apparently, been used by Entwistle in his tour rig. It cost £99.
Don't have it any more though... :-(