Sure, a slider can make sense there, I agree. But now I've got a part of the screen dedicated to be the spot to tap to start changing the volume and a part dedicated to it being the brightness taking away from the other useful parts of the screen, or its hidden under a sub menu making it more annoying to rapidly change.

Imagine if on your phone to change the volume you had to swipe into a settings menu first and then change it on a slider versus just using the volume buttons on the side. Seems like a worse way for something you're potentially wanting to rapidly adjust, like when you accidentally start playing something way too loud.

Why not have the "buttons" change the touch bar itself into a slider when pressed?

If you locate them in the middle it could be a seamless "press, drag, release" action