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.