With exception to single window utility programs, Mac windows have always truly closed with the resources taken by the represented document being freed and all that. The windows weren't hidden. It's just that closing the window ≠ quitting the application… the program can remain in memory even if it has no documents loaded.
This serves a couple of purposes: first, documents open more quickly (particularly when the program is loaded from a slow spinning HDD, floppy, etc) since the program doesn't need to be reloaded, and second, new document creation flows and non-document functions can be accessed without having a document open or requiring the developer to create a bespoke "home screen" UI that serves that purpose since the full menubar is accessible as long as the app is foregrounded.
It's just a different set of expectations. The original versions of the Mac OS should almost be thought of as a multiple-document interface. Consider the web browser you're reading this in. You wouldn't expect closing a single tab or window to quit the whole application, would you? That's really what was going on in early Mac system software. Go to infinite-mac and open Mac Paint on a System 1.0 machine. It becomes very obvious when you open the app, and all of the Finder windows and desktop icons disappear.
This is only confusing in comparison to Windows though. If you used graphical DOS applications, it was the exact same experience. You open the app, and can interact with your documents, but closing a document doesn't necessarily close the app.
Even Photoshop on Windows of the day worked the same way. When you opened Photoshop, a parent window would open that was the app. Closing documents left the app open, unless you also closed the parent window.
The comparison to modern browsers is odd and IMO plays into GP's point. You can't get a modern browser to be a single process so it is like your examples and bad for it.
This serves a couple of purposes: first, documents open more quickly (particularly when the program is loaded from a slow spinning HDD, floppy, etc) since the program doesn't need to be reloaded, and second, new document creation flows and non-document functions can be accessed without having a document open or requiring the developer to create a bespoke "home screen" UI that serves that purpose since the full menubar is accessible as long as the app is foregrounded.