They're not. C++ isn't a low level language by any stretch of the imagination.
Javascript isn't either.
Both of them have a glut of good cross-platform gui options.
With mono, C# and winforms work pretty well cross-platform too.
What do you call Python, JavaScript, Java, or C#?
Python isn't first-class in any GUI framework IIRC (although Qt for Python may change this). JavaScript and C#, with regards to GUI, are single-platform languages (I've never seen a UI built with Node and no browser, and I don't see a good way to build a Linux GUI in C# that is actively maintained). That leaves Java, which I will concede, even though Java isn't first-class on any platform except Android.