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.
I almost never have Finder open. I usually think "I want to create a spreadsheet" so I'll open Excel and then look for the document. Or I'll be doing some dev work so I'll navigate there in the terminal and then open code via `code .`
I think some of that habit might have stemmed from the fact that I don't like Finder much. I don't like the hotkeys (eg [ENTER] to rename), I don't like it's layout. As much as I disliked Microsoft's "lets turn explorer.exe into Internet Explorer", the editable address bar is actually really nice (you can see precisely where you are and quickly change it to where you want to be).
I'd say Dolphin (KDE) is probably the perfect file manager for me. It's got a terminal baked in (which automatically changes directories when you navigate in the GUI) so you can mix and match GUI and CLI operations. You can add tabs so your file manager can be used as a session (eg if you're working in multiple directories for a specific task or project, you can have one file manager instance open for that task but different tabs per directory). There's so much I love about it. But it's also the polar opposite of Finder, which might explain why I don't like Finder much.
I do completely agree with you about Gnome though.
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.