MDI was rightfully seen as a complete failure, but there was also SDI, where each open thing is a separate window. I don't know how we got from MDI in office apps being completely terrible, to MDI in browsers being the accepted norm.

MDI in browsers isn't enforced though, you can still use each tab in it's own window, and (depending on your browser) you can open other sites in a sidebar (? Used to be possible) or split-tab (I think I've seen this in some desktop browser? I know chrome does it on mobile). Office documents though? SDI is now enforced on office. You can't hide that taskbar entry for every single document you have open, even though there's only 2 important ones each open in their own window and the rest could all share a window for all you care.

MDI wasn't a failure, the requirement was a failure. SDI requirement for office is also a failure. More so than the escapable (multiple window support) MDI of browsers on desktop.

Actual MDI was so much worse than browser tabs, unrelated tabs can be merged into the same window or split apart into their own, instead of floating on top of an awkward background.

The question is why aren't they a feature of the window manager instead of the application. We should be able to have windows with tabs from different applications.

Tabbed MDI is effectively just a better interface to SDI (for most situations)

Actual MDI applications feel so dated. It made more sense when there wasn't a unified task bar kinda thing (which when you think of it, is kinda like tabs as well)

Well websites and documents are not the same thing so it makes sense that a paradigm that works for one doesn't necessarily work for the other. I do find web-based document editors very annoying to use when they are in the same window as other tabs - at least web browser MDIs allow you do effortlessly separate tabs into a new window these days.