Why manufactures don´t burn in their amps and ...

IF burn in were required, just think of the facility required.
1 station with good power for each amp along with dummy loads appropriate to the power. One good power glitch and there goes a couple months of inventory....poof!
The heat alone would put a dent in ANY electric bill. Unless of course you did all your burn-in during the winter!
Bryston does so simply for infant mortality reasons and in order to provide for the #1 industry warranty.
Also, I have read some posts where people periodically re-burn cables!!
No mention is made of the other half of the equation....that your ear/brain gets accustomed to the sound it hears.

This I believe: components don't burn in; listeners burn in. "Burn in" is a metaphorical cousin of "break-in": baseball gloves require breaking in; conert grands--specifically, the action-- require breaking in. The wood in string instruments changes. We love to think of things breaking-in, balancing, aging into maturity and ripeness. We like to think of metal components, non-organic and w/ no moving parts (excluding drivers), doing the same. Isn't it odd, though, that "burning-in" is supposed to result in better sound? Isnt' that very congenial on the metal's part? Why should that be? We imagine things "limbering up," "getting settled." Just as I like to imagine that my tube amps give me a warmer, more open sound. I suspect that I'm transforming the literal warmth--the visible glow--of the tubes, and the delightful, illuminated open space inside the glass globes themselves, into my perception of the sound. Solid state. I mean who wants to hear music from something called Solid State. Definitely not open, and not warm and glowy.

here here

why did manufacturers used to make power cords that were hardwired, and millions of music nuts didn't give a ratsass about burn-in? jeez were we simple?..or simply happy with the equipment? UL approved red cords always sound better than UL approved black ones, and blues ones always trump the red. throw in the words 'hospital' or 'military' and your electricity will really pay attention.

I listened to my system for a year with the stock power cord.
I decided to try a new cord and it did make a difference.
A window in my room began vibrating/rattling.
Never did before.
Some frequency was boosted and made that glass vibrate.