The macOS open/save dialog is almost identical to the Finder UI, what would you change about it? The reason it's helpful to be able to drag a file from somewhere else into the open/save dialog is that you often already have the file open somewhere. A common workflow: navigate to a document in finder, edit it in some app, save it, upload it to a web form. Similarly if you have a path to the file handy somewhere, you can hit ⌘⇧G and paste it.

And while I can see how reasonable people can disagree about the virtues of Mac/Windows/KDE open/save dialogs, Gnome definitely has the worst. Personally my KDE experience is limited, but of the other three I prefer macOS by a wide margin.