Why no “Break in” period?

jea don't kid yourself. The manufacturers have to remain politically correct or people won't by their equipment. You have to be a personal friend to know what they really think. 
Noromance, I did not make up that expression but in most relatively minor cases it is quite true. Obviously not if your child gets killed in a car wreck. 
Accommodation is used as a medical term and you are right. Everyone is accommodating all the time to numerous stimuli. 
All those guys "burn in" their equipment primarily to make sure they are working up to snuff and nothing is going to blow on the customer. 
Having said all that, just think about it from a manufacturer's standpoint. The customer calls because things don't sound just right. The easiest come back is, "well you have to break it in." Withing a few days or weeks the customer gets use to the sound and things are peachy keen. He thinks the unit broke in when in reality it is he that broke in. The human brain is way more flexible (and unpredictable) than an amplifier. Many companies now put something in their manuals like, "don't worry if it sound like crap. It takes three months for it to break in." When people don't know why they make stuff up. The Greeks made up Zeus. A very creative explanation for lightening.
Anyone care to give us just one proven reason why a purely electronic unit's sound should improve after manufacturer's burn in?
My ESLs break in because the diaphragms are initially over tensioned by a heat gun at the factory and it takes about 100 hours of play time for them to loosen up to the proper tension. The factory does not do it for obvious reasons.

Actually it’s very difficult, for speakers, to separate out the effects of diaphragm break-in and the effects of break-in of internal wire, speaker cable, internal crossover parts like inductors and capacitors. You know what they say when you ASSUME something. It makes a fool out of me and Uma Thurman.

Thanks for all your thoughts related to my question.  I really do appreciate all the knowledge you all pass on.

"expectation bias"

?
This is practically a standard of movie reviews. How many times have we read about the reviewer coming into the movie not knowing anything about it? Or how the movie was nothing like the trailers? Or everyone said wow best movie ever, changed my life, another Marvel, the first two Guardians were great, this Star Wars was "supposed" to be, and so on.

Its a standard thing, right? Everyone knows what I'm talking about, right?

Which means you also know the very next thing to follow the expectation is the BUT and the HOWEVER or every once in a while the AND IT WAS. Right? This is something every single person on the whole freaking planet understands. We all know this, right?

Further beating a dead horse, driving the point home, like a stake into the vampires heart, we all know that the reviewer is perfectly capable of then writing about his actual experience of the movie IN SPITE OF EXPECTATIONS!

Sorry about the all caps but when you have someone so unbelievably dense they can't understand what everyone else on the planet just automatically gets its like all sense bounces off the skull until you're shouting in vain to get through. Probably other side of the impenetrable skull is vacuum. Let's see if it can learn. 

Don't hold your breath.

"Bias" is bias, regardless of the viewpoint or subject.     ie: Those that are so adamant, regarding their beloved theories/opinions/biases(regardless of the source), while refusing to acknowledge that ONLY experimentation(the heart of the Scientific Method), provides PROOF, regarding anything discussed.     Most of those are proffering their opinions, without ever having tried what’s being discussed.     What you hold true, in your listening room, is all that matters.    Experiment and trust your ears.       Anyone that discredits another’s abilities to hear improvements, in their own systems, in their own listening environments, with their own ears, should be considered condescending, insulting and/or(probably), simply projecting their own ineptitude.    Perhaps, to be pitied.