Moving to InDesign EPUB forum
It isn't included because it's not required to create a valid EPUB 3 fixed-layout document. There are other platforms that support FXL that don't need that file in the META-INF folder. Adobe decided a while ago not to "cater" to proprietary ebook selling platforms (e.g., no option to export to MOBI/KF8). It's very aggravating, I agree! You might want to submit a feature request here: Adobe InDesign Feedback
In what cases are you finding that manually adding that file solves your interactivity issues? I am curious.