The problem is always one of backwards compatibility.  They can include them in OS7, sure - but that's so limited in usefulness to anyone who targets multiple platforms.  We still need to include a workaround/alternative solution for the platforms that don't include the libraries by default.

I kind of wish they went this route (external classes/libs) for more components, it would make it much easier to manage building  consistent interfaces across OS versions, without having to reinvent the wheel several times.