I think this is a valid point. For example, I have always wondered why default browser styles don't at least adjust to the OS's theme - be it Metro, Material, Qt, Gtk, whatever. It would solve a lot of issues for bare-bones websites, while still allowing full configurability for websites via override. And if we really cannot have nice things because of backwards compatibility, make OS integration optional via <meta> tag etc.
Why does every website have to reinvent the wheel?
The problem with browser default styling is that there should be another level: user default styling. I.e., the styling as chosen by the user.