MacOS is the only one with a concept of proxy icons. It has a distinction between the key and main window. There's a responder chain for event handling. Toolbars have a built-in editor. There's QuickLook for easy previewing of sub-files. There are standard popovers, which can be torn off.
On the Windows side, I'm sure they've got their own set of unique features and constraints that the Mac doesn't have. (The Ribbon and keyboard navigation come to mind.) Same with Linux.
Lacking a feature often means you need to design the UI different elsewhere. Without proxy icons, you might need to add a menu item to mimic that functionality, for example. These things domino into each other. There's a reason that good Mac apps and good Windows apps don't tend to look identical.
English and German are fairly similar, as languages go, but you can't just do a word-for-word replacement of English and get a good German paragraph, or vice versa. Not infrequently, you need to re-arrange everything to make it work. The pieces you have to work with are just different.
2D/3D graphics APIs (if that's what you're referring to) aren't like this. For the most part, they're all playing with the same basic pieces. When my Mac traded OpenGL for Metal, I barely noticed, even as a developer writing graphics code. When my Mac switched from Carbon to Cocoa, everybody needed to rewrite almost everything.
To port the application to Windows, you’d lose a lot of the Mac-specific features, or have to do a substantial amount of work implementing platform-specific UIs.
There's a long-running subculture in MacOS (and once Nextstep and now iOS) software development that I'll call 'app craftsmanship'. At its best it produces high-quality, slick apps that usefully leverage the platform's strengths and build dedicated (and typically paying) user bases. So when someone puts out a new Mac or iOS and makes a fuss about how it's fully native and supports the Face Bar™, they're signaling to potential users 'Hey! I'm about to do some App Craftsmanship here, people' and hoping deciated users will flock to them, credit cards in hand. They're not selling to the people who are going to be irritated the app doesn't run on Ubuntu Maimed Mastodon. And that's not a moral failing, it's just market targeting.
After all, there are good Windows- and Linux specific applications as well, and for most fields learning a new tool for doing things you understand isn’t that hard.
Your example is how I used to develop portable native software as well.
