My theory is that designers target users viewing 1 full-screen window at a time. Their model user never moves or resizes a window, or opens more than 3 tabs/documents a day, as they sit at their 13" laptop in an ideally-lit room with no reflections.
Then you get programs having splits or panels or whatever they're called for multiple documents, which is just an internal window manager that's not as good as my regular window manager.
It's impossible to see where one windows frame/border is, sitting on top of another. This should be an OS thing, but apparently it's an app thing.
And VS Code "withdrew" support for configuring borders: https://github.com/microsoft/vscode/issues/160159