Simple question, probably stupid

Tubes don't burn in but your hearing gets accustomed to the music and tricks your brain into believing there have been changes. Interestingly, literally every example that I've read of something "burning in" has resulted in better sound. How could that be?

Mho, there are no stupid questions, really.
Inspired to inane answers, yes. 

I've viewed 'break-in' as something that should be done with ss as well; 'conditioning' to a life of service...

I recall when for vehicles of any sort called for a break-in period of +/- miles before one did 'pedal to the metal', to let all the mechanicals 'seat 'n seal' properly.

How about a 'structured approach'?

10 hr. 'idle'; on with no signal....just warming up.
10 of white or pink noise, low volume.
10 of low volume program, 10 of louder, etc.

That allows for some degree of 'grats', even if not 'immediate'.... ;)

Can't hurt, might do the thing that's suggested without having to 'pause the Cause' for 100... ;)

Enjoy the 'arrival', J

I have always found that tubes do get better with "burn in time". As others have said and as Brent Jesse who is my go to for NOS tubes says you have 2 options: leave the amps on for the desired period of time, no signal needed or just burn them in by using them. I've generally done the latter as I really enjoy hearing how they change. 

+1 @larryi Just listen to your system, you’re wasting the hours on your tubes. 

I guess I’m lucky. I’ve never had tubes sound bad right out of box. Having said that, some tubes get better in a few hour.  It’s not hearing bias or my ears adjusting.  I usually by an extra set when buying tubes,( just in case ) and have A/B them after a few hours and they do sound better.
 

All the best.