What makes sense is really just a up and down, or a knob. Volume isn’t something most users can judge visually.
Actually, you don't — tap the "button" and just start sliding your finger. Same for the brightness control.
I think it is one of those cases Apple sometimes has where they come up with a good interface, but people are so used to bad ones they don't even realise that a better way might work.
I'm not surprised that I've seen a lot of complaints about the touchbar; if you're used to physical keys, a touch-screen is no substitute. (Fortunately, I was already weaning myself off Fkeys, so I don't really miss them.) But I am surprised that I haven't seen more discussion of what it gets right. A slider that you can actually slide is a good UI metaphor for volume (as opposed to tapping fake keys repetitively). The touchbar also can show buttons from dialog-boxes, letting you "type" them instead of having to move your hand to the mouse. It isn't always used to great effect, but it's clear that some real thought was put into it, and I appreciate the attempt to take advantage of what it can do.
That works (sort of) but in a way it's even worse. Now you're sliding your finger a couple of inches to the right of the actual control image, further damaging the illusion of a physical control.