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

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. 

In the final analysis all we can listen to is manipulated power. But it is manipulated power,  be certain of that.
Is it even possible to listen to precisely the same signal every time, even if we wanted to?
Conversely could we possibly hear marginal differences, even  if we wanted to?

After my philosophical bs, 
Let me agree re tube 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......