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

It's critically important to "burn in" any component.

The initial burn in period will just exceed the length of time during which you would have been allowed to return the component.

The remainder of the burn in period will just exceed the length of the manufacturers warrantee.

No conclusions should be drawn regarding the performance or reliability of any component until fully "burned in".

Shorter return periods, warrantees and/or egregious restocking fees will all shorten the length of time it takes to burn in a component.

Cost, pure and simple. If tubes are involved, the catch would be that 'used' would need to be put into the spec's.

With exception of speakers, the 'burn in' is a myth to get you into the rituals of the hobby. Double blind test, the only true way of telling, has proved time and time again it is not necessary.

Most performance vehicles do require driving at limited RPM's for the first few thousand miles of seating the pistons. Porsche's is about 2000 miles. I would guess that manufactures don't break in audio gear because it takes 300-400 hours of constant running sometimes within the context of being within a system with a signal running through (speakers and amplifiers). Ultimately it probably just doesn't make economic sense. OTOH, I see no reason, given the modest cost of materials involved, for any high-end manufacturer to provide "better" power cords. Again, I'm sure the reason is economic in that the more they can keep manufacturing cost down the more appealing to the consumer, the better it may look in the face of competition, and the more potential profit. It's the kind of thing where if one manufacturer starts a trend, other's may follow.

If they include a power cord, they would not be able to stay at a competitive price for there gear. Also, many audiophiles already have a favorite power cord that they would use in place of the stock cord. I believe that a good power cord can make detectable difference, as always this is just my opinion and we audiophiles have been debating about power cords, speakers cables, and other tweaks for years, its just a matter of opinion.

Most factors/dynamics(with the exception of speaker system drivers) that are involved in "burn-in" will also diminish with idle time, and require another burn-in period. If a manufacturer turned out 2000 amplifiers, and ran them the 200hrs, to their peak performance(burned-in) level, he'd then have to charge you a percentage of the electric bill. AND: By the time the unit was sold; (in most instances)It would require another burn-in. A waste of time and money(though SOME do burn-in their products). There are many differences in peoples' tastes regarding their music presentation, and various presentations that can be had via different power cords. How would a manufacturer know if what he provided would suit your tastes? Most of us would be rolling our PCs, still looking for that last drop of resolution, sound stage, impact, silence(or whatever) anyway. Why should they bother? Then there are many that can't hear a difference(regardless of how audible and obvious, to those that can), or are so convinced it won't make any difference; they refuse to hear any. I can easily understand why most manufacturers don't bother with burn-in or providing upgraded PCs.