This is completely silly, and no one does this. If I'm going for a square design, I'll want my checkboxes to be square, not rounded squares like Firefox offers for example.

Try to extend this idea to a game menu and see how much sense it really makes to you: would anyone really want default browser checkboxes in an otherwise medieval styled game UI, for example?

Also, if you truly believe that all checkboxes in all apps should look exactly the same (per browser/OS, and except for color), then why not extend this to other aspects of design? Why even allow different sites to use different fonts? That is much harder to adjust to than a different roundness on checkbox corners.