Because software is generally written to run on a specific platform and Word and Excel, like Windows are Microsoft products and the whole of Microsoft Office was written to run under the Windows operating system.
Thanks, we might change “Words and Excel” to independent, like “Windows”, and I think that will have two advantage: Microsoft can one independent production and we can get more choice, if we don’t like use “Windows”
I think that is easy to get, we can offer special source code for “Words and Excel”, so we can install that “Words and Excel” in computer like “Windows”
It's not quite that simple unfortunately. Whilst source code is theoretically transportable there are many components of complex packages like Office which depend on low level access to the platform on which they are designed to be run so there are technical reasons. In addition to that many authors have a vested interest in limiting the packages on which their products will run. Your final point about "offering" source code only happens on products with "public licenses" whereas products like Microsoft Office have their source code jealously guarded. I'm afraid the world we live in is commercial rather than the utopian and philanthropic world which you understandably would espouse.
I think source code of window is not so complex, it is just two simple figure, that two simple figure is consist of different circulation for help people enter or exit application software, as you know window is platform for application software
The source code of “Word” and “Excel” is more complex then source code of Window, but it is also so simple, but it has different circulation