Why does it take so many hours to brea in arc preamps and amps?

Make two reasonable assumptions.
That there is some of legitimacy to the notion of "getting accustomed to the sound" burn in.
There is probably an element of the circuit undergoing real physical changes "during/over use" time as well.
Thus there is perceived 'burn in.'   QED....  EOD...... 

To Randy11

Who do you think you are insulting members? Like you own the forum, or are vastly superior in intelligence than the rest of the members? 
Make a positive, beneficial comment or dont at all if you just want bash people. 

DM

Actually, an expert in court provides expert opinion or evidence, not proof. See the difference? Besides an expert can be wrong, his testimony doesn’t guarantee success for whomever he is testifying. And experts don’t always agree. That’s why the defense expert witness often disagrees with the plaintiff expert witness. Hel-loo! That’s why the Appeal to Authority is an illogical argument.

what was the insult darkmtter?

No one can deny that Nelson Pass is an expert, and the experiments prove him right.

Or are you a science denier?

Authority is not as good as an on point experiment but a LOT better than an ill informed opinion?  If you don't believe that then do your own surgeries and find out.