When the touchpad is set to emulate a mouse scroll wheel while using a circular gesture, swiping clockwise is scroll down and swiping counter clockwise is scroll up. Of course, you can change this if you want:
Steam>Settings>Controller>Pair and Manage>Desktop Layout>Edit>Edit Layout>Trackpads>Left Trackpad Behaviour>[gear icon]>Invert Swipe Direction
Or just swap the "Clockwise Command" and the "Counter Clockwise Command" under Steam>Settings>Controller>Pair and Manage>Desktop Layout>Edit>Edit Layout>Trackpads>Left Trackpad Behaviour
But regardless of the fact that it can be changed, there must be some logic to why the Steam Deck touchpad behaves that way by default, right? Help me make sense of why some dev team at Valve decided that the default should be "clockwise=down" despite these other common scenarios in which "clockwise=up".
You're comparing apples and oranges. Volume is a value without implying any kind of progress. Time is progress, and turning clockwise gets you further down the progress of time. A scroll bar going up is something visually going up, but it's not a value, and it's not a measure of progress going forward. If anything, it's going back, because you usually start at the top of a list or page that requires a scroll bar. So progress here would be scrolling down.
But I think any interface should make this sort of behaviour easy to configure.