You gotta keep in mind Word/ Excel (in fact all programs of their size) keep a lot of temporary files. That won't change and you can't specify that. They do it for speed and reliability. Imagine if a write gets corrupted or interrupted. Since it is writing to the original file, that is likely to corrupt the original file. So pretty much all programs write out to a temporary file. As soon as that is completed, they delete the old file and just rename the old file. Office doesn't rename it because nothing is worse than working with only one copy of a file. Microsoft was tired of hearing about how "Office corrupted their only copy" and "We've lost MONTHS of work."
So they've made an temporary copy of your working file ever since.