Part of the appeal may simply be an UX thing. For instance, I don't think a PDF reader should be a part of the browser codebase, but when I'm reading papers I found on HN, I like it that the PDF opens as another browser tab, not a separate application window.
I think browsers shouldn't really have tabs in the first place. That's the job of a window manager. And then you can decide which one to use. Do you want tiling or floating windows? Do you want workspaces? Stacked, tabbed, side-by-side layouts? Hotkeys or UI for navigation? Performance or features? And so on.
I agree, but browser developers have to work with the reality that most users use Windows, and windows has no reasonable solution to handle tab equivalents, or even large amounts of windows. And windows doesn't have a good way to extend or replace the window manager to fix that.
So instead we are stuck with each program implementing their own tabs.
