One reason might be accessibility: for some people, it's a lot easier to hit a hardware button than a tap-target on a touch screen. You can feel around for the edges of the button, you get clear force-feedback when it's pressed, and you don't have to worry about accidentally hitting something else nearby.

It would be quite easy to improve both the "accessibility" and the practical UX of capacitive touch screens by establishing a norm where every potentially-destructive action (anything that cannot be "undone" with ease) can be made to require a 'slide to confirm' gesture. This workflow is already used by community-made "recovery" environments for touch devices, but it would be nice if it was adopted more widely.

So you know what buttons you're pressing, Same reason people hate the apple touchbar.

That sounds like an argument for a button-centric device, not using the volume rocker on a touch-screen-centric device.

You can hold the phone AND use the buttons at the same time, with the same hand.

I frequently use my phone one-handed. Easier with smaller phones, which is part of why I prefer them.