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

It took well over 300+ hours for my REGA Isis valve cdp and REGA Osiris integrated amp to settle in .... the actual burn-in process was longer but less pronounced farther out on the time vs performance curve. At 300plus hours, that timeline point was something of a common threshold that apparently is not unusual for many different brands.

Studies by cognitive psychologists show that sensory experience alters perception.

So, is it the tubes, the caps, or the listener?

perhaps whoever recommends 600 hours break-in to any component except i guess car or boat, definitely doin' some substance abuse...
it might be either mushrooms or lsd.

My .02$, elements within tubes do change with use.   Frequently this is perceived as a diminution of harsh, hard, bright and othee unpleasant qualities. It shouldn't take a billion hours to get rid of this, sometimes only 60 hours, but more is typical oh let me say on average, oh just guessing,  for a typical set up maybe,  output tubes about 120, but please this is obviously highly highly variable.  Don't come back at me telling me this amp takes waaay more time or whatever. If your experience is different I would not be surprised.   
Two things. 1 tubes continue to change to some extent throughout their use, but usually reach a level sound.
2.  AND in ARC gear this can be very important.   You don't hear the tubes in certain circuits.  I had an ARC  pre and I  rolled 12AX7s in it couldn't hear any changes, tried a bunch of 5751s, same effect.
As for caps, That I don't know and of course  the circuitry itself ages.