Because window manager is hard to extend and hard to customize . I can trivially change this and lots of other things in Firefox, but with windows WM i would ha

Because window manager is hard to extend and hard to customize . I can trivially change this and lots of other things in Firefox, but with windows WM i would have to wait 3 years to get any bug fixed or a new feature added. Also calling chrome or Firefox window "just a holder of tabs" isn't fair, they are very special holders of very special tabs, and lots of work goes into them to make them work properly. Also there are other "holder of tabs" windows, e.g Sublime Text, which are mostly similar but have important differences. Trying to move all that complexity into WM simply won't work.