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

The simplest answer is that it's a business, you want all of that done for you, then you are going to pay for it! You aren't getting it for free. Component makers are not cable experts and are not about to squander revenues to research for the best cable for their component. Whether you like it or not, that's your job!

I would think most manufacturers have some type of a QA process where the product is burned in for x number of hours to weed out infant failures that occur.
Some may even have enviromental chambers that control temp,humidity,and vibration that affect the product.
Power cords buring in may have some type of merit but I don't know what the audible improvement would be.

Rleff- Once you've purchased a hi-end power cord(ie- Synergistic research AC Master Coupler, PS Audio XStream, Zu Mother- or better than those mentioned), and burned it in; you will understand the changes through which it will progress(the electrically biased ones excepted). That depends on the resolution of your system and your ability to discern the differences(system variances and aural acuity/training differences existing in so great a divergence).

Rodman99999- I did purchase a set of kimber power cords don't remember the model but it does not have a network on the cable and will be installing them on pair of audio valve challenger 180 monos I just picked up on Audiogon and am looking forward to hear their performance vs the signal cable cords I had before.
Do you think it is worth putting a high quality ac cord on my soundlab m2's backplate?; Let me hear your point of view.

Rleff- I used to own a pair of Acoustat Model IIIs, and was just recently wondering if I would have heard any difference with some of the PCs I've got now. The voltage that electrostatics use is charging the speaker's diaphragm/membrane, and not providing gain to any kind of signal. That's one of the very few applications in which I believe would be hard to discern an improvement(No experience/just a guess). Then again- I've heard that Martin-Logan CLS's can be improved via upgraded PCs. You could plug those Kimbers into the SoundLABs,(before using them on the monoblocks) as an experiment(I would).