Why would you want your browser to manage themes and window management (tiling, links outside of web) while it is the job of the OS/desktop environment?
Part of the reason is that this is a macOS browser. macOS's window management is a joke. But I think I'd use the tab tiling on Linux too, I do a lot of work with tiled browser windows and it's great, but also sucks when you want to have a window very small horizontally and the browser's UI gets squished along with any extension menus you're trying to use. Arc doesn't have that problem, you always have access to the full UI.
Arc specifically has that problem. I use a tiling window manager on macOS (guess what, they exist!) and Arc doesn't play nice with it, specifically because of the vertical-only tab bar.
Doesn't that argument apply to tabs also? Eg IE6 days where there were no tabs, and everything was deferred to the OS/desktop window management. It was clunky and painful.
I consider that an indictment of window management, not browser UIs.
I used [0] to use EXWM + qutebrowser in tabs_are_windows [1] mode. Being able to switch between tabs/open files/IRC rooms/whatever else emacs can do with fuzzy buffer searching felt like computer enlightenment.
[0]: EXWM runs in emacs's normal lisp environment which sometimes blocks the thread that EXWM listens for events on... this is obviously untenable and I never got around to finding out how to avoid this
It should. Browser developers started introducing tabs because OS like Microsoft Windows and Mac OS were lagging in term of UX but that doesn't mean this is where it should ideally go. This was just bandaid.