Apple had one person who was good at UX. And that guy is dead for years now. His product pipeline can't be milked further to make more money.
From here onwards its just your usual managers running the show.
Woooow, this is a revelation. Thank you!
But to answer your question: It's hidden because it doesn't have a graphic representation on the device itself. Nothing on the volume button indicates that it's somehow a hidden slider.
The whole point of a good UI is you shouldn't be reading a manual and watching videos to do X, Y and Z.
Apple used to be a pioneer at things like this.
Sad things have come down to this.
At "Touch Bar shows".... change that from "App Controls with Control Strip" to "Expanded Control Strip". Then you will see the individual buttons. You can then further "Customize Control Strip..." and you will now see more buttons. But you will also loose "App Control".
Of course, why can't those specific buttons be a customization option there with "App Control with Control Strip" forcing you to choose between volume up/down and "App Control"? Because Apple.
For fuck's sake... Thanks for the "upgrade", Apple.
Hah, You couldn't make this stuff up... GUIs were borne out of a hatred of highly context based keyboard behaviour (VI, Xerox Parc), now Apple go full circle and revert to context hell by combining a keyboard with a touchscreen GUI. Goodbye Apple, it was nice knowing you.
This same UI is found on other touch screens including my Honda's dashboard.
I also accidentally 'touch my pinky finger when typing my password (numbers) and it registers as touch even though I'm not applying downward force. In Chrome thank god I could remove the refresh button, would accidentally refresh login pages all the time.