I wonder why Google does not track the time to an event fired on another Google Analytics site. While it might yield strange results if the user closes the brow

I wonder why Google does not track the time to an event fired on another Google Analytics site. While it might yield strange results if the user closes the browser or opens a page without Google Analytics, it would provide quite accurate results. Of course, this only works if most websites use GA. Also, I am not a web developer, but is it not possible to specify another GA event in the onbeforeunload function?