Windows (IE) tried to give WM visibility into tabs long ago, but this quickly revealed a problem: if tabs are now also separate windows, they enter the Alt+Tab ring, and suddenly you're Alt+Tabbing between 5 applications and dozens of open tabs. N=1, as a tab hoader, I absolutely hated it (it also made Alt+Tab itself very slow to show and cycle).

The general problem is that having WM handle in-app tabs means the WM has to support every single hare-brained idea about meaning and role of tabs that app developers invent, which is infeasible. Even with browsers - until recently, you had like just two types of tabs: regular tabs, and porn-mode (er, "private browsing") tabs. Then came "account containers" or whatever it was in Firefox. Then came tab groups, reading groups, and tons of other nonsense.

OTOH, I can dream of an ideal world where the OS manages all tabs for all apps via combination of Sidebery (better "tree style tabs") and a tiling WM.