Same reason why you might prefer to use an app over a website in general. I use PWAs a lot on Windows, and for me the single biggest benefit is that they don't clutter my browser tabs, but instead show up as separate icons that I can Alt+Tab to, see in taskbar along with their notifications, move and resize the window as I see fit etc. Sure, I could also run them in separate browser windows, but then window management gets much messier.
But I don't prefer using an app over a website in general. For me, the biggest benefit of tabs is is that they don't clutter my task bar or my workspaces and that I can ctrl+tab through them.
What does it even mean to "clutter" a taskbar if you exclusively use websites? i.e. what is the point of a taskbar devoid of apps other than the browser?
In-app tabs are supposed to be the second level of grouping, hence why it gets its own shortcut. What you're describing is an attempt to flatten that hierarchy, which is a valid approach, but why are you surprised that not everybody shares your enthusiasm for it?
