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.

here here

First, there are NO high frequency 'surges' in a power cord.
If a power cord measures as highly reactive, you will have voltage and current not in perfect phase.
Just for a trivial example for which I have measures.
A 40 watt fluorescent light draws about .32 amp after warmup. The power factor (PF) is about .8, so the lamp really draws 40VA.
If the PC is such a load, along with the powersupply of the equipment in question, that can be the source of some bad effects.
The electric company really hates low PF loads, and at least in industrial applications, charges a premium.

As an aside, the same thing can be said of the amp/speaker relationship. A speaker with huge phase angles can suck the life out of an amp while having only a fraction of the power delivered to the load. Add low impedance and the problem compounds.