The problem is it's all of those things. For a text ad, the creative is the words. For a display ad, it's the imagery + text. For a YouTube ad, it's the video. Content is the other stuff on the page where your ad is being shown, so "creative" has become the standard term to encompass the stuff in your ad that you make.
Except "marketing content" is a whole other discipline, around things such as blog posts and the like. Not to mention "creative" has been used by advertising agencies for decades now...