One possibility for the wxWindows project would be to write a generic program using the wxWindows API. Unfortunately, this would result in a program hundreds of KB in size, competing for disk space with the program that is to be distributed. Also, since installation needs differ widely from platform to platform, it makes sense to code each platform's installation program individually.
wxWindows Install has therefore been written in native Windows, and is consequently around 41 KB in size.
The INSTALL.INF format is not an original file format. It is based on an existing installation utility file format, but I don't think that the format itself is copyright. All the code is original, written from scratch with no reference to any other code, and some extensions have been made to the file format to allow enhanced functionality.