I think it is because they invented tabs. This window grouping proved to be useful. KDE has tabs but I don't think there is a way to say "open a new window tabb

I think it is because they invented tabs. This window grouping proved to be useful. KDE has tabs but I don't think there is a way to say "open a new window tabbed with this one" so simple functions such as middle-click to open a link in a new tab are very hard to implement. macOS was sort of closer here with grouping windows by application but it didn't provide good management options and you couldn't regroup manually. So browsers implement their own tabs and so browsers are basically becoming a second window manager. Once you have this it is natural to consume more "applications" to fit into your window manager. The web is a powerful enough feature to start this process. I wrote about this a while back https://kevincox.ca/2021/01/11/tabs-were-a-mistake/ https://kevincox.ca/2021/01/11/tabs-were-a-mistake/