Why...
Very simple since if you use the office products in a professional capacity with a lot of automated integration of meta data that drives other references in the documents, and I haven't even started about macros/vba, you have no choice.
All the other products will change the advanced layouts, if you are lucky don't strip out the meta data but will mangle up the document references.
I love pages, but for example you can't even have a document title being referenced in a header or a foot. So if you change the title of a document, you have to manually change all it's uses and now search and replace is not useful for that.