Whistle clean

I don’t understand the phrase “Clean as a whistle.”
People put their mouths on the things, and they’re full of spit.
The corks in the whistles end up with all kinds of mold and bacteria in them.
Just awful.
Unless it’s a new whistle, I won’t put my mouth on one.
Even if you bleach the damn thing. Twice
It’s like toothbrushes. Or re-using someone’s dental floss.
Or unwashed underwear.
It’s just something you don’t do.
Throw that shit out and get a new one.
That’s what disposable means.
As long as you’ve got a new clean one, that is.