If you had a dozen pairs of classic vintage ubes in your closet

Why would I have tubes, that I've never heard?        Regardless of how satisfied I am with whatever tubes are currently in use; I know there's always room for improvement.      The only thing I've never been able to resist is temptation (especially: to experiment).

Are you asking why people don't replace worn out tubes? Or are you asking why people don't feel compelled to audition tubes they own, even when the tubes in use are fine?

The usual is to try them even if satisfied with the currently installed ones. You never know.
That's what i did when had plenty of nos tubes, now most of them are sold and couldn't be happier.

G