Burn-in and Equipmemt Value

A lot of people strongly believe that burn-in results in better sound quality and some will even pay more for cables that have the burn-in done by the manufacturer. If burn-in is real, then why isn't used equipment worth more than it is? At a minimum, shouldn't the demo equipment from a respected retailer be worth more when manufacturer warranties are still in play and the equipment is essentially new?

As a side question, why is it that any perceived change in a system where burn-in is credited it is assumed that the burn-in was on the newest piece of equipment? Some users report changes from burn-in hundreds and thousands of hours down the road.

I understand break-in on speakers and tube amplifiers, but struggle greatly with things like cables and digital sources.