I agree that the reason does not matter and is irrelevant to the end-user. I was just speculating as to why they have not "just fixed it". However, I disagree that Windows is more polished. Windows is made up of a mixture of UI frameworks/controls/apps, some going back decades. Much of this legacy is poorly integrated and even basic things like drag-and-drop doesn't work consistently. At least Gnome is consistent and they have a vision.
If there are issues you'd like to see fixed, the best way to get those dealt with would be to contribute.
I would very much like to see the file chooser/ open and save dialog in GTK fixed because it affects a lot of software even if one is not using Gnome.
It affects Firefox on Linux on all DEs, it even affects Gimp on Windows. I am not a GTK/C/Vala programmer though.
I would like for gnome apps to behave better when opened on DEs other than Gnome.
Other than that, I don't really care about Gnome. I use Plasma on Linux.
Well, if you want some changes made, you would have to define what you mean by "fixed" and come up with some concrete steps to do that, and then implement those steps, or find a person who can do them for you.
If the app uses GtkFileChooserNative, it can likely already be configured to use an external file dialog, with some caveats: https://old.reddit.com/r/kde/comments/a5cxwk/firefox_v64_can...
If Windows has issues with paths longer than 260 chars (only PARTIALLY fixed since Windows 10, v1607), I do not care that NTFS actually has no problem and supports 32,767 Unicode characters and the actual problem is the Windows API (and since v1607, apps that have not been updated to support the new behaviour). It is an example of a bug/limitation/lack-of-polish of Windows. The internal details are irrelevant.
Gnome has a lot more of these issues(thumbnails being one of the most visible and annoying ones) than Windows. Gnome (any ver) is not more polished than Windows.