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.