Because the Windows headers are so much bigger. The platform specific distributions already contain the generated wrapper code. The bigger the header, the bigger the resulting wrapper code.

No, EWG does not support C++. It only supports C. C and C++ are two distinct languages and C++ is far more complex. WrapIt is a tool that tries to wrap both C and C++ libraries. WrapIt is not a fully automated wrapper generator as EWG is. It is my understanding that WrapIt does not parse the C/C++ header directly but instead reads an interface file, that the user needs to provide.

There are a couple of areas where EWG doesn't have all information it would need to properly satisfy all C compilers. This is being worked on and the warnings will eventually disappear.