I have a little programming experience and I did get to work with Swing (Java) and wxPython. 
Some years ago, I ended up having to port an app I did to OS X and all the problems I've had with wxPython just multiplied. I've started looking at alternatives and I could not find anything better than wxPython. QT was the only one that came even close. GTK was hell on Windows (last time I checked). 
I don't think my predicament is unique, some other people might have reached the same position over the years.
So, why isn't there a GUI toolkit to have the following characteristics:

Simple. Basic windows/dialogs (native open and save) + a basic set of widgets with SOLID Layout management. Additional (composite widgets) implemented as additional libraries. 
Actively maintained on OS X, Linux and Windows
Actively maintained bindings to at least 3 high level languages. 
Actively maintained documentation for best practices GUI devel with examples using the toolkit. MVC/MVP properly explained. GUI HIG (Apple style) simply explained, complete with semantics, paradigms, best practices, alignment, proper spacing and TONS of best of their kind examples from successful pieces of software.  
A simple way of building a distributable executable for the 3 targeted platforms in each of the languages. 
Implemented as close to the metal using readily available libs like Clutter, cairo, etc. 

Are these requirements too much to ask? If yes, why?
Help me understand why isn't there such a tiny lib available?
What you ask is a simple lib with enormous flexibility. Be able to build 3 * 3 executables, having good documentation and using other libraries all with a different background. And all of that on different OS platforms. It is quite a challenge it think QT would com the closest but it does not meet all your criteria.
Why are there no simple, cross-platform, C/C++ GUI tookits for higher level languages?