Say, how come the manufacturers do not burn-in their audio devices before shipment? If a product is certified as being ready for your enjoyment, then one is more likely to prefer that one over "here is a headphone, I hope it works". I suppose a sealed speaker enclosure which has been shipped in an aircraft might need air pressure equalization much like your ears or water bottle after a trip. But headphones.. I don't have any theory to explain it.
And let's take this a couple steps further. Suppose burn-in is real. Manufacturers could then start selling "factory certified burned-in" pairs for a premium. Why don't they?
Also, I find it curious that manufacturers don't publish their data on burn-in. Most pairs go through a few years of R&D before release. Didn't Sennheiser put about six years into the HD-800? One would think the drivers have been run for tens of thousands of hours in testing and measured quite carefully. If burn-in exists, then it would surely turn up during R&D. If so, then I'd imagine the manufacturer would either include burn-in instructions with the headphones or burn-in at the factory.
Beancounters and lawyers don't like products that change during the warranty period. That opens up liability and potential returns - those are very expensive. I can't imagine any manager approving the shipment of a product that would substantially change. That's just too risky for a business.