It helps to keep in mind where the OS and its core user base is coming from.
For the case of the up button for example, prior to OS X the Finder was a spacial file manager where each folder had a single corresponding window that remembered its position on screen, allowing users to rely on spacial memory to quickly navigate filesystems. Its windows didn’t even have toolbars, because they weren’t navigator windows — every time you opened a folder you got a new window (unless that folder’s window was already open, in which case it was foregrounded).
So when OS X rolls around in ~2000 and switches the Finder to navigator windows, they’re looking at what existing users will find familiar. Back/forward is easy since most had used a web browser by that point and map cleanly to most people’s mental models (“I want to go back”), but up? That’s a lot more rare. A handful of folks who’d used an FTP client might’ve been familiar with the concept, but few outside of that few would’ve, and how “up” relates to a filesystem is not in any way obvious. And so, the Finder never got an up button, just a key shortcut because anybody advanced enough to be hunting down shortcuts is going to understand the notion of “up” in a filesystem.