Its also a lot of work and harder to maintain - I’ve built web-apps like this and inside the DOM there is no inherent (read: Good) window manager - there are some libraries out there, and there are oddities between different browsers in those libraries - I rolled my own and it fuckin suuuuuuucks to write and maintain events through the windows (in my case the windows are movable iframes with borders)
Rant aside - my point is yes this may be something in future, but it is a BIG job and with existing competent cross platform management tools already available it would not (and should not) be a priority for their developers.