Because any directory can contain subdirectories, the file system can be arbitrarily deep. This organization of nested directories and files is called a “hierarchical” file system. Again, though the advantages are obvious in hindsight, hierarchical file systems were not widely available before Multics and then Unix. For example, some file systems limited the depth of nesting; CTSS limited it to two levels.
— Brian Kernighan, UNIX: A History and a Memoir
If the idea of a filesystem limiting the depths of nesting sounds insane, imagine a filesystem where every level of nesting is conceptually different, where there's one program that you use to manage some levels of nesting and another program for other levels of nesting, where you can't move things from one level to another.
That is the state of window management today:
Different levels of nesting: workspaces, windows, tabs.
One program (the window manager) that you use to manage the first two levels (workspaces and windows), another program (the browser) that you use to manage the next level (tabs).
You can't turn a tab into a window, or a window into a workspace.
Yes it's a mess, as confirmed by operating system vendors' constant struggle to fix it: Windows, tabs, views, widgets and mini-windows, along with Exposé, virtual desktops and now Stage Manager are all symptoms of the same issue: dealing with unlimited hierachies with limited screen real-estate.
An unlimited screen wouldn't really solve much. The real limitation is our working memory – a cognitive problem. Visuals are there to offload working memory, and when you find yourself having three different windows of the same folder, you know visuals has failed at its job.
Surely Stage Manager might give us an extra "cognitive percent". Improved visuals, cues, an extra metaphor here and there might give us another percent. But at the end of the day, I think we reached the end of the road unless we rethink the concept of files, folders and the "jurisdiction" of applications. So at some point down the file hierarchy, arbitrary applications take over and visualise the rest of the hierachy in whatever unorthodox, material style of the season. However a browser window is a container and its tabs are locations, i.e. files in some aspect. A bookmark is also a file. The Photoshop pane of Patterns is also files inside a container. And since the hierachy is hijacked by applications, browsers must afford their own ways of sharing content besides the operating systems sharing mechanism.
To add to the mess we have Open and Save dialogs, which is just another unneccessary way to visualise the same file hierarchy, with their own means of navigating and creating new folders.
My $0.05: Let applications visualise and edit files, and leave the handling of hierachies to the next generation of operating systems. That way the nice patterns in Photoshop could display as a pane in the operating system hierachy since the operating system leaves the rendering to Photoshop (if it's available). Tabs doesn't have to be tabs. They could be displayed as vertical lists or thumbnails, however the content of each tab thumbnail would be rendered by the browser. A container of tabs (really "bookmarks with a state") could easily be moved to another application or browser.
"You can't turn a tab into a window, or a window into a workspace."
If I grab a tab in Safari, Firefox, or Chrome and drag it outside of the tab bar, it turns into a new window. If I drag a single-page window into another window's tab bar, it becomes a new tab; I can also drag a tab from one window's tab bar into another.
I hate Illustrator's single-window multi-tab mode so I keep it off but it works similarly. I can also full-screen that single window and there are some controls for arranging multiple sub-windows across it. Photoshop works similarly, some of Adobe's other programs also work similarly, some have no concept of even working on multiple documents at once, much less multiple windows/tabs for them. There's a whole workspace management layer inside AI and PS and programs like them, though. Lots of janky proprietary tiling window managers crammed into a single executable. I've been trying to learn Blender lately and I have no idea what its tiling window manager is inspired by but it sure works nothing like how I expect stuff to work on my Mac, for instance.
MacOS has tabs as part of the windowing system; I can drag from the tab bar of one Finder window to another to turn them into a multi-tabbed window, and drag the tabs back out. This seems to be an OS-level feature that must be enabled by the program, a lot of apps don't. Does Windows do this? Does your favorite Linux window manager? I have no idea. As far as I know, tabs first appeared in a now-dead web browser named Opera and quickly spread to other browsers, then other apps, then to OSs. If you had written a browser or Photoshop or whatever and added tabs before the operating system supported tabs, would you want to bother rewriting your tab system to support the different ways you have to talk to the window managers of MacOS, Windows, and/or Linux?
Some programs are simple little tools that can fit into a complex workspace. Some programs are entire workshops that offer workspace management. The state of window management today is a *lot* more complicated than you think it is.