(In reply to joshas from comment #4)
Why not use native menus on other platforms too?
At least on Windows, because win32 APIs do not provide any "nice" menus, and Windows 10/11 itself is confused about what menus are supposed to look like.
I can't speak to the situation on Linux; we use some of the relevant gtk/x11/wayland APIs, but I don't know if there is a higher-level linux context menu solution that we "should" use that supports all the features we need. For sure it'd be a significant investment.
Currently context menus on Linux are looking out of place, considering main menu has rounded borders, but context menus were left with straight rectangle corners.
This is bug 1706378.