OS window managers do a better job of that. Split view inside the browser has some thorny issues around making sure the user knows what resource they are interacting with. There is a lot of complexity when it comes to focus/blur in HTML, CSS, JS, etc.
Unpopular opinion: Tab management in browsers originally addressed the shortcomings of OS window management (see Windows XP and IE6, and the original Google Chrome tiling capability replicated into Windows 10/11 OS window management.
I also share this opinion. I think we are in a local maximum with tabs. But getting out of it request a lot of coordination between browsers and each desktop environment so it is unlikely to ever happen. Maybe less portable browsers like GNOME Web or Safari that only "need" to deal with one desktop environment can manage it at some point.
Tabs state management is simpler and more battle tested. Split pane browsers will need to relearn some of the same problems/security found when tabed browsing was introduced. They will have unique problems/security as well. I would be interested to see how split pane browsers deal with focus stealing JS especially with timeouts or other shenanigans.
