So although basic access to Qt class signatures and methods is not difficult, being able to provide access to all the capabilities Qt provides in a manner that is well-suited to the language requires more than that.
This is no different with GTK. Getting good idiomatic bindings requires more than just using the introspection, you need language-specific code that maps toolkit-specific concepts to the language in question. Try comparing pygtk's original gobject property handling, which involved parsing a dictionary with strings, to their modern method and decorator-based approach. That didn't happen automatically, it had to be specifically programmed (as PyQt4's property system did, although it doesn't look like they ever used the ugly dictionary method).