Many advanced window managers on Linux have support for tabs on WM level (so you can combine arbitrary windows into tabbed containers etc).

The problem is that even if you have a browser with a single tab, you still get the browser tab bar showing that tab. And then on top of that you get the WM tab bar, wasting space and confusing the user.

In older days, a browser with a single tab wouldn't show the tab bar at all. But that has changed ever singe they shoved the tab bar into the window title bar on most platforms.

I still want to open multiple websites in the same browser session, I just don't want them all to live in the same OS-level window so I can use my window manager and scripts to reorganize and quickly switch between them